Typically the problem here is the bearing in the clutch. If the squealing stops after 1-2 minutes after a cold start, then it most likely is this bearing. There are threads on replacing this bearing, a chinese one that is easily available. I went the other route and bought a rebuilt air pump. I am starting to get a little noise already and it has only been a year or two, so this bearing isn't the best. Either that or the rebuilt wasn't done very well.

1991 300CE
2000 C280