Rear shock mount failure is a very common problem, and can occur in as little as 20k miles. The symptoms begin with a dull clunking noise in the rear over bumps or rough roads, indicating that the shock piston rod has separated from the bushing mount. This can progress into metallic noises as the mount bolts shear off if not replaced in a timely fashion. The broken mounts eventually damage and tear the rear shock towers. But all this is can be avoided if you just inspect the towers and mounts periodically. Stronger E46 mounts along with Z3 reinforcement plates are recommended for replacement, and even better aftermarket parts are available from outfits such as Rogue Engineering and Ground Control. I used the standard E36 mounts, and have had no problems yet in 1yr/8k mi. Like I said, just check every so often.