My 1995 Mercedes C280 makes a long, high pitch beep when the climate control system is turned off and I ease off the gas pedal. When I speed up, it stops, when I slow down it starts again. If I turn on the air or heat, it stops. Any ideas?

All of the A/C control flaps are vacuum operated. Possibly one of the vacuum elements are leaking(squeaking), the fresh air intake element is a common failure.

I've seen a few of these caused by the blower motor. You can try removing the blower moter, which is pretty accessable, and put a few drops of 3 in 1 oil on the exposed parts of the motor shaft.......Gilly