A common item that requires replacement in E36 M3s are the Vanos units.  These vehicles are single vanos engines, varying the intake cam according to the requirements from the DME.  The newer models (E46 cars) use dual vanos, varying both the intake and exhaust cams.

Over time, the helical piston and helical internal gear of the variable cam sprocket begin to wear, creating more and more noise.  Some may sound like marbles, loose nuts, rattling, or even a "gargling" sound.  This is normal for vanos to sound like this, but again, as it gets worse, it gets louder.

(click to enlarge picture)
Cam sprocket and Vanos shown seperately	Cam sproket mount on Vanos

Submitted by Ben Liaw