My 500E has developed a mystery noise that two mechanics and an exhaust shop have been unable to identify (and they are getting annoyed by the fact that I'm not letting this drop).
Here are the symptoms:
1) When the car is rolling, there's a noise I can best describe as the kind of high-pitch squealy/chirpy noise that a library cart or utility dolly with un-lubricated wheels would make. A sort of warbly, garbled, squealy, chirpy noise. I wish I could upload audio of it.
2) It's pretty audible outside the car; it can also be heard inside the car. Best heard at low/moderate speeds (like 15-30mph) though I've noticed it on the highway too.
3) Seems to come from under the car, mid-line
4) Happens almost exclusively when giving the car throttle (especially light to medium throttle; heavy throttle not so much) -- if I take my foot off the gas and let the car coast, noise stops immediately
5) EXCEPT when rolling to a stop, while moving like 2-3mph, you can hear the noise again even if foot is off the gas as the car rolls to a stop
6) Applying the brakes has no effect
7) Car MUST be moving -- revving engine while standing still yields no noise
8) Also, car has to be WARMED UP for this to occur -- usually 15-20 minutes after starting to drive; once it appears, it stays until car sits and cools for a while.
So: rotational chirpy noise that's much more pronounced when giving it gas gently while car is in motion; can also hear it at very low speeds while coasting to a stop even if no throttle is applied; no noise when revving engine while stopped; audible both inside and outside car; car must have been driven for 15-20 minutes at least for the noise to appear (and once it appears, it stays).
Car's performance doesn't seem to be affected. Yet.
I'm thinking this might be a something having to do with the differential, given that it's both movement- and throttle-related. But it's not the kind of gear whine I'm familiar with from previous run-ins with a bad diff, nor is it the kind of droning that I'd expect from a bad wheel bearing (nor would I expect a wheel bearing to care whether or not I'm giving the car throttle).
The car is pretty much in perfect shape otherwise, so this is driving me a little batty. Anyone who can help me solve this will have my eternal gratitude (and a round or two of drinks on me, if you live in the Bay Area and feel like meeting up!).