[Mnjdnght]

I had a weird experience recently and would like to understand potential causes.

Part 1 - I was driving from Hyderabad to Aurangabad in my BMW X3 30D on Feb 6. On the smooth Tuljabad - Aurangabad 4 lane sector I did some reasonably quick driving in Sports mode for about an hour or so and then eased off to a lesser cruising speed. At one point, with an empty road ahead and around, while I was at about 110-120 kmph or so, I heard/ felt two back to back thud sounds from the car. It sounded like the pop sound when compressed air releases or, as my wife felt, something soft hitting the car (but twice!). I also felt that it came from the rear right side of the car. I was attentive on the road and was pretty sure that I didn't go over anything. My first suspicion pointed to some issue with the tyres. I pulled over and checked. Couldn't find anything wrong with the tyres or wheels. On board vehicle status also didn't show any faults. With no further idea on what could be wrong, I decided to drive on and look for any other issues while driving. But it was a smooth drive, no loss of balance, alignment, power or mileage. Rest of the journey was uneventful.

Part 2 - This occurred on the last leg of the same trip. I had reached Pune from Aurangabad and was on my way back to Bangalore from Pune on Feb 13. After a lunch stop at Chitradurga, I was again doing some good speeds for an hour or so, this time not in sports mode, before easing off the pace. This time I hear a much louder, I am tempted to say explosive, sound. I am sure I am explaining the sound clearly - It was loud, scary and distinctly audible. But it was not a hard sound (like crackers) but a softer one, maybe like the sound when planes hit a major turbulence all of a sudden. It was quite startling on an empty road where I am sure I didn't go over anything and no other vehicle was around. This time too I felt it coming from the rear right side. It felt like highly compressed air popping out or something big and soft hitting the car. Though I've no prior experience with a tyre burst while driving, I suspected a tyre burst to be my first guess. I was near the Guilalu toll gate and pulled over near the toll gates. I couldn't spot any issue with the tyres or wheels nor could I see any evidence of anything having hit the car. On board vehicle status also didn't show any faults. The car started and I decided to continue driving. I drove back the rest of way at a very sedate pace and reached home without any further incident. The car handled well and there was no hint of anything wrong during the remaining drive.

I am not able to come up with any explanation based on my little knowledge or imagination about what could've caused these sounds. I don't even know if I can replicate it, and even if I could, it may still not be possible to simulate similar conditions. I had started my drive from Bangalore to Hyderabad and that drive didn't face any issue and neither did the drive from Aurangabad to Pune.

I did some research on google, with vague search strings I must admit. But couldn't figure out anything much. Some remote possibilities seem to be related to issues in exhaust system or related to injectors/ timing etc. but I don't know how to trouble shoot that line any further. I spoke with the service centre and they, for now, don't suspect anything wrong with the car and asked me to observe once more during the next drive but I would be a bit reluctant to do one without some understanding of potential candidate causes.

In my mind Part 2 was a more serious repeat of Part 1 and am very concerned about how a Part 3 escalation of the issue would be like, given it is happening at reasonably quick speeds.

Looking for your help in getting some understanding of this issue, ideas or possibilities or anything at all, before heading out with the car again or taking it to the service centre for an inspection.

My car is the 2016 model, stock, with no mods and still on its first set of Pirelli RFTs, though soon due for change. I bought it in Feb 2017 and has run about 45K KMs so far, over 90% on highways. So far the car has been running fine, except for a bad fuel issue which got resolved without incident, and serviced on time under BSI+.

To the Mods: I had posted this in the Technical Stuff sub forum (https://www.team-bhp.com/forum/techn...ml#post4751481 (What's that sound?)) but I feel it may not have been the right place for the topic. Hence starting a new thread as I would really love to get some help in understanding this.

[SS-Traveller]

Originally Posted by Mnjdnght

...I did some reasonably quick driving in Sports mode for about an hour or so and then eased off to a lesser cruising speed. At one point, with an empty road ahead and around, while I was at about 110-120 kmph or so, I heard/ felt two back to back thud sounds from the car.

...I was again doing some good speeds for an hour or so, this time not in sports mode, before easing off the pace. This time I hear a much louder, I am tempted to say explosive, sound. I am sure I am explaining the sound clearly - It was loud, scary and distinctly audible.

...I would really love to get some help in understanding this.

Seems like you heard the sound of backfire from your exhaust pipe(s). A fouled catalytic converter can cause this, and it would not be a surprise if the catcon has been damaged due to poor fuel quality, considering the amount of highway driving you have been doing (presuming that fuel quality from pumps on highways is suspect).

Is your car passing an emission test easily? You might want the service centre to check emission, and clean out the catcon if required.

[Mnjdnght]

Originally Posted by SS-Traveller

Is your car passing an emission test easily? You might want the service centre to check emission, and clean out the catcon if required.

Good point. The last emission test was done in September 2019 before my Goa trip and is due for the next one soon. My first brush with fuel quality happened during that trip after which I've been more aware and careful. I've not done an emission test after I've started experiencing these issues. I am taking the car for a check up later this week and will have this checked out. Thanks.

[2000rpm]

A few questions

1. Are you on stock tyres? If not, upsized tyres can touch the wheel well at high speeds bumps (used to happen with my Swift)
2. Did you check the boot? A lot of times, things we assume as rattles are loose objects.

Also, humble suggestion, do not stop and check the vehicles on secluded highways, especially of the car is behaving normally. These highway scams are scary and make me really worry about travelling with family on long distances. I take flights for all travel more than 5-6 hours.

[GTO]

Mod Note: Thread moved to Technical Stuff!

[Mnjdnght]

Originally Posted by 2000rpm

A few questions

1. Are you on stock tyres? If not, upsized tyres can touch the wheel well at high speeds bumps (used to happen with my Swift)
2. Did you check the boot? A lot of times, things we assume as rattles are loose objects.

Also, humble suggestion, do not stop and check the vehicles on secluded highways, especially of the car is behaving normally. These highway scams are scary and make me really worry about travelling with family on long distances. I take flights for all travel more than 5-6 hours.

I am on stock tyres, but due for a change soon. Mostly will retain same specs.
Being on a long road trip, I did have a lot of things in the boot, but checked and made sure none of those were the cause.

I 100% agree with your suggestion/ caution. I made sure I didn't stop immediately but moved to the left and crawled along until I found a safe-ish place to stop, a petrol bunk in instance 1 and the toll gates in 2, all the while checking the mirrors and around to look for anyone else showing an unusual interest in our predicament.

[keroo1099]

Originally Posted by SS-Traveller

Seems like you heard the sound of backfire from your exhaust pipe(s). A fouled catalytic converter can cause this, and it would not be a surprise if the catcon has been damaged due to poor fuel quality, considering the amount of highway driving you have been doing (presuming that fuel quality from pumps on highways is suspect).

Since you said it happened when you backed off the accelerator I tend to agree with SS-Traveller that it most probably was a backfire.

[Jaggu]

Apart from what has been already posted.

Get the car to the dealer and get it checked, if something shows up in diagnostics you have a lead. Other things to check: suspension, drive axle, prop shaft, and wheel bearings. Exhaust backfire can be a reason, but on a stock car and that too on a fast highway where the exhaust system would open up (even if there is a mild block), chances are rare.

Did you feel any jerk or impact type of thing during the second instance?

[Reinhard]

If you are suspecting bad fuel then one of the cylinders misfiring may also be a cause that induces backfire in diesels. Dirty injectors/deposits on gaskets or rings... There are plenty possible root causes that can cause a misfire linked with shady fuel.

Ideally these should register as error codes and be visible on the OBD report though.

[Santoshbhat]

Have you checked the inside sidewall of all the tyres? Sometimes bulges on the inside part can stay hidden from sight. I'd suggest unmounting all 4 tyres once and having them inspected just to rule out

[Maverick5490]

Have the mountings for the transmission and the rear differential checked, maybe they are weak and shifting post hard acceleration and slowing down.

[Mnjdnght]

Originally Posted by Jaggu

Other things to check: suspension, drive axle, prop shaft, and wheel bearings. Exhaust backfire can be a reason, but on a stock car and that too on a fast highway where the exhaust system would open up (even if there is a mild block), chances are rare.

Did you feel any jerk or impact type of thing during the second instance?

I was going at a steady speed when suddenly I heard this loud thud/ pop (sort of a muffled explosion). I didn't feel any jerk nor did the car swerve/ change direction. Have booked an appointment for Friday and have created a checklist of items based on suggestions here to go through with them.

Originally Posted by Reinhard

If you are suspecting bad fuel then one of the cylinders misfiring may also be a cause that induces backfire in diesels. Dirty injectors/deposits on gaskets or rings... There are plenty possible root causes that can cause a misfire linked with shady fuel.

Ideally these should register as error codes and be visible on the OBD report though.

After a fuel quality related incident in Sep 2019, I've been very careful in selecting fuel stops, the one before the second incident being the Shell after Hubli bypass. But I won't rule it out as a possibility. Maybe, an earlier fill could be suspect. I am also hoping that the OBD report will provide some clues.

Originally Posted by Santoshbhat

Have you checked the inside sidewall of all the tyres? Sometimes bulges on the inside part can stay hidden from sight. I'd suggest unmounting all 4 tyres once and having them inspected just to rule out

I've not yet checked the inside part but will be doing that at the service centre on Friday.

Originally Posted by Maverick5490

Have the mountings for the transmission and the rear differential checked, maybe they are weak and shifting post hard acceleration and slowing down.

Thanks. Will have this checked too.

[Sandeep500]

I think SS-Traveller is correct in his explanation about the cause of problem . As your car is about 45000 Kms driven things in exhaust like catalytic converter might be blocked with soot and carbon deposits. Also the EGR valve also needed to be cleaned. Due to continuous high speed driving the self cleaning process might be going on in the exhaust chamber by burning of soot and carbon . So when you might have eased on throttle a bit there might have a explosion inside exhaust chamber. And as it made of high quality steel and various other layers inside the sound herd was soft instead of being shrill.

[Mnjdnght]

Visited Navneet Motors on Hosur road today to check this up. In addition to their own trouble shooting based on my problem description, I requested them to check specific items as mentioned in the posts above. However, the actual cause still eludes! Nothing turned up in diagnostics as well. They also took the car for a high speed test drive and the technician who did the test drive didn't experience the slightest issue - but I told them that given how it manifested for me, it may not show up in a short duration test drive. Looks like whatever caused it has not left any evidence behind.

During the process, they observed that one of the engine mounts has worn and the drive shaft has a slight play both of which will be fixed under warranty. I am fairly sure this is unrelated to the problem though.

I'll be continuing to explore/ investigate further and will keep the thread updated with any finds.