Why Do Malfunctioning Cars...
 

...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!

Are you sure the rattling sound isn't coming from the air pump which is only engaged for a minute or so after a cold start?

Sixto
95 S420
87 300SDL
83 300SD ... $950, see cars forum

check serpentine belt tensioner. that can cause a nice rattle first few seconds. I just had that replaced on mine.

Keep a tape recorder in the car, record the noise, play it to the mechanic. The sound may be all the clue he needs to solve your problem.

-Bryan