I just went out to run errands in my '87 420SEL and experienced a brief, high-pitched squeal several times upon acceleration from a complete stop. The sound was more like a squealing wheel sound, not like a squeaky belt. Does anyone have any ideas what this could be? For info, I have 73,000 miles on it and I wasn't running the air conditioner. Any response would be greatly appreciated.

Make sure your fan blades aren't hitting the shroud.

I notice it in my '99 C280 too
However, mine was whenever I try to accelerate over 2500 rpm, I get those high-pitch noise and it doesn't sounds like it is from the wheel...
Please comments on it too....

Someone said it the telltale signs of the smog pump failing on this forum. Do a search on the subject.