My 240 started making this infuriating noise just yesterday, so I'm resorting to you folks for feedback. It is a whining noise, much like a slipping belt, only upon closer inspection I found it comes either from the rear part of the engine or the transmission (automatic), not the pulleys. Also, it'll only do it when I come to a stop (say, sitting at a red light) and the transmission is engaged. If I shift to park or neutral, it goes away. I did find a few posts describing a whining noise coming from the driveshaft, but they all said it would whine when accelerating, not idling.

I have the similar sound from my tranny, the problem was the ATF low level. Inspect the ATF level with the warm motor.
I hope this help you.

Edwin

Are you familiar with the sound that a hydraulic pump makes when it is failing? Could that be it? That sound is most commonly heard when a power steering pump is failing, but I have heard a similar whine coming from an automatic transmission.

If that is the sound, this won't be a do-it-yourself repair, unfortunately.

Try adding RPM w/foot on brake and car in drive, if noise changes you probably have a clogged ATF filter.