Team-BHP - Weird Click-Click noise in rear tyre of city zx

One thing that I can now see is that this is a widespread problem. Well.... and I am sure it comes from all cars. Even my new Indica makes this noise from the rear tires. And this has been there since the very beginning. Somehow I never thought it serious enough to worry about it. As I realized though is that the sound is more pronunced / I realize the sound only when I am passing by a flyover or a divider or a wall. Anything that reflects the sound back at me.

I have never been able to figure out why it always comes only from the rear! Also, what exactly makes this sound. One tentative reason I feel is the mud flap. It actually flaps due to the draught of air that rushes backward and hence makes that tic-tic/ cluck-cluck/click-click noice. If you take a small semi-rigid rubber piece and try to fan it I think it will also make almost the same noise.

ok people..solved...there was a huge..and i mean huge screw stuck in the tyre...which was causing the noise..the screw was as big as around 2 cm in radius...the noise is gone now....
thanks a lot guys...:-)