Well once I though about this, I had a direction.

The squeeling started when I changed the fan belt. I went down to my MB dealer, paid 20% more for a fan belt and presto, problem went away. The Continental belt I bought from Performance Product 4 Benz was bad. I looked at the belt and it looked fine. For me, the belt will be a part I buy from the dealer.

steve, my 98 mercedes developed a similar problem at just over 100,000 miles. it took me a while to hunt down the culprit, even though I am a above average mechanic. I determined it was my serpentine belt tensioner vibrating, thus making the belt chirp. I ordered a new style tensioner And installed the new style tensioner vibration damper. ( a small stabilus/bilstein shock absorber) my old tensioner didn't have the mount for the damper. the other end of the damper connects near the thormosat housing on my car. i took photos while doing the job if you want to see them. anyway the end result- no more vibration, therefore no more chirping.