Car is a Tip 996tt. I've been encountering a clunk at the rear sometimes when I accelerate from a low speed (around 30 mph at 3rd gear). Mechanic said the engine/tranny mounts are tight. The subframe is ok. The sway bar and U/CV joints are all ok. The "thump" doesn't happen all the time. Mechanic was unable to find the problem. Any idea?