I have a 1992 300TE wagon that sounds like a bird chirping at times. Sometimes it is at idle, other times I can hear it at 15 to 30 mph. It does increase in frequency when the accelerator is pushed. It does not happen all the time, usually when my wife is driving it (but I have heard it too). Is it just a fan belt in need of replacing or something else?

Any hints would be greatly appreciated. Thanks!

I have had dirty and worn brakes causing a chirping sound on the 280SEL before. I'm not sure what exact sound you are hearing, but I thought I would throw in my $0.02.

Chirping sounds coming from the engine, usual are caused by the multi-belt. You can check this with a candle(light)...push the candle(light) against the inner side of the multi-belt whilst the engine is running. The wax of the candle light will get rid of the chirping sound.
This solution is (usually) only good for a short time and the chirping sound will probably come back within a few days/weeks. By the you know for sure that it is the multi-belt causing the sound and you will have to find out whether it is caused by a worn multi-belt or a bad pulley.

greetingz,

I had exactly the sound you describe on a 190E a year ago. If you carefully run dripping water on the belt with the engine idling, it helps isolate the source. I changed the single belt tensioner, cheap and simple to replace. Once the old one was off, I found the centre bearing shot ! Good luck !