hi every one ,can some one help me diagnose a knocking sound coming from my front wheels,especially loud when going over bumps,sounds like brake callipers shaking my mechanic had a look and couldnt find the fault although he dismissed problems with the exhaust,would like to thank you for your time and appreciate your thoughts.

Get a different mechanic. That sort of noise from the front of an E39 will be worn suspension parts or something else loose in the front (motor mounts, subframe, etc?). It should be pretty easy to find.

Thrust rod bushings are shot.....Even if they are no split look to see if they are
leaking. It will be evident by a whitish black stain on the bottom of the bushing.