I had this happen on my '90 420SEL. It turned out, in my case, to be the air
pump that runs at engine startup for a minute or so to dilute the exhaust
gasses for emissions purposes. At least this is my understanding of it's
purpose. Anyway, if the pump is frozen, and the clutch engages, the belt
will slip and thus cause the squeal. My solution was to unplug the connector
going to the pump.

John S.

You need to change the belt.

my friend had the same proble turned out to be serpentine belt adjuster
stoped adjusting rubber mounts inside unit had failed the belt slips
when you start up and can squeel if you pull away hard .

Check tension on belt

It may also be the bearing of one of the small pulley’s (one above the
alternator and one on the tensioner).

I have a very annoying noise at startup also, but it's not a belt, or
anyhting else that I can think of that would be on an average car.

It is the most annoying noise in the world and it sounds like it's coming
from the glove box or under the right front speaker.

It sounds like this
aeaeaeaerarrarrerereaerrghhhagreaerrarhagerhaeghrgeaehraerhagehraeaghr !!!!

Like when you were a kid and blew up  balloons and then pinched the hole and
let the air out to make that god awful squeal....

Well that's the noise it makes. It's just at startup, only when it's cold,
and it last for 10 to 60 seconds.

Any thoughts on what That could be ??

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