I have been hearing an intermittent chirp/squeak from my 1991 Mercedes 420 SEL with 560 motor for over a year. It is getting worse now but It still is not consistent and never happens when the car is stopped. It only happens at speeds of 45MPH and up. At first I thought it was the heater blower but it is not. It only happens for a few seconds at a time. Belts were just replaced. any troubleshooting advice?

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

failing belt tensioner

Thanks. I will have my mechanic check.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

Did you mean the A/C idler pulley. If so, that was replaced.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

for me it is the belt tensioner. I /just/ finished replacing it an hour ago. same symptoms

I have noticed on this car of mine I recently purchased it makes a squeaking or mousey sound intermittenly. To my hearing it acts like it's by the driver's side door, but thats just a guess. It's almost like it is a timed sequence. Is this possibly the same as you are in reference to?

Rebe's issue seems to describe the bottom of the door trim rubbing on the plastic door sill covers. This can be caused by a slight tire imbalance and so seem mechanical. This is well-known W126 trait and has been discussed elsewhere in this Forum; try a search.
Other sources of squeaks include accessory belts, worn pulleys, front sway bar bushings, basically anything that moves can be a potential source of a squeak.

Thanks for your replies. I replaced all of the belts and the A/C idler pulley. It seems to be sensitive to the throttle. When I give it more or less it seems to stop the sound. It does sound mechanical like a screeching pulley but it never happens when the car is stationary which is making it hard to track down.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

I searched the threads and found some info on the door sills etc. that can cause squaking. I will try some of the remedies and maybe resolve the squeaking noise. It is not that disturbing, but you can definitely hear is happening.