Why Do Malfunctioning Cars...

[avatar218]

...behave perfectly in the presence of a mechanic? A couple of weeks ago while adding (yet another) quart of oil to my 300SE, I heard a distinct metallic rattling in the alternator, like BBs rolling around. (So far, the car starts OK, and the dash light hasn't come on). Right now, the car's at my mechanic's place for a few days and while there, it's been acting perfectly normal. Anyway, more than once lately I've definitely heard what sounded like a loose alternator bearing -- but if it won't rattle in front of my mechanic, how can it be diagnosed and fixed? Is there anything else that could be causing the rattling sound? Thanks!