Car (88 M6) drives beautifully now (new radiator, thermostat, AFM, FPR), but has a sort of "rattling" noise when I let off the clutch. It seems pretty harmless, just very annoying. My mechanic says something about using a different gear oil (synthetic?) but didn't really touch on it (he and I both agree I need other things first). However, I would like to make sure this isn't a serious problem, and also like to see how much I'm looking at to fix it in the future (what the heck is it?)