@Anurag I didn't find anything amiss driving in fourth at speed of 45. I generally up shift from 3rd while crossing 40. I always play by the ear while driving and shift based on the sound of engine. I have found that to be a better technique than sticking to manufacturer quoted gear to speed ratio.
Coming onto the solution to the problem actually it just struck me that the rattling is due to play between some parts. I had never seen the mechanism below the lever so it was a speculation. When I reported the problem at service they took me on a TD after their so called rectification and when asked how they solved it they replied by putting grease.
Now the problem did subside for some time so I deduced that grease had worked but must have failed due to it getting liquified at higher temp in fact grease will flow even under normal sun.
So the hunt began to find something that retained its consistency and lubrication at high temperatures. The solution was to use silicone grease since it is thicker, stays where you put it, doesn't stain or leave an oily residue, is colourless and odourless and most importantly can withstand temp upto 400 deg celsius. This same grease is used to lubricate O rings for high temp and pressure applications.
I got hold of high vacuum silicone grease of wacker company one of the best as per reviews and went back to the ASS. I asked the SA to use silicone grease instead of normal grease. He was skeptical but agreed. This time I was present while they carried out the job. Sorry didn't click any pics as I wasn't sure if it would work.
The technician first removed the plastic bit onto the left of the hand brake while sitting on the driver seat and then slowly he pried out the entire black panel from below the arm rest to just below the cubby hole. You can't remove the panel until you have unbuttoned and loosened the Velcro of the boot cover even after that there is a cable attached to it for parking brake indicator which if required can be disconnected after this both the shifting and selecting mechanisms are visible.
The following things exist for me as a layman:-
A. The gear lever sits on a kind of ball and socket joint which is mounted on an aluminium frame.
B. Towards the engine side of the joint there are two plastic arms which are attached to the sides of the lever one of these carries the selector cable and the other the shifter cable.
C. Both the cables are connected to the arm through a spring loaded telescopic mechanism which can be adjusted to control the free play. A plastic lock is present which stays in place due to spring tension.
D. I was appalled at the shifting and selecting mechanism as they seemed very fragile.
I found that whatever grease the ASS had put had shifted location and also melted leaving oily mess but for the love of god since there is no beige coloured plastic or fabric there everything and every bit of the grease was removed using cotton waste. Once I was satisfied with their cleaning I handed him the silicone grease it was about 25 grams in a jar. The grease was applied at the ball &socket joint, each and every sliding and swivelling plastic part, on the springs, the telescopic mechanism attaching the cables( the cables are not visible) after application the lever was slotted into all positions and it was confirmed that all nooks and crannies were covered. unlike conventional grease silicone is thicker and requires a thin layer and being transparent it can be ensured that it has reached the intended places. The panels were refitted and all purpose cleaner ensured that no technician handiwork spots were visible. The ASS wanted to go for a TD which I refused as was short on time. On the way back I tried the same 4th gear method and the rattling was gone. I have driven a few Kms after the fix, the rattling has completely gone and even if I keep my hand on the lever vibration is no longer felt as Tanveer mentioned. Moreover I have found the gear shifting to be more precise and less notchy also earlier there was was an audible sound during slotting which has also reduced considerably.
My verdict:-
Anyone facing the issue to go for this fix blindfolded as it has made my life easier as that nagging rattle is gone for good. It can be a DIY but not for the faint hearted as there are no screws to remove the panels, it is all snap fit and one forced tug may break a lock or two and marr the fit & finish.
The gear slotting is much better than before.
As regards the silicone grease it can be easily sourced saw a few tubes available on Amazon or can be procured from industrial or auto parts supplier.
All the best and goodbye rattling