I had an annoying rattling sound coming from my windshield area that I first thought was due to a loose wiper (strangely, my right wiper sometimes gets hung up on the lip of the hood instead of retracting behind it as it should -- so the wiper is definitely loose), except that turning on the wipers did nothing to change/eliminate the noise. So you may want to try that to make sure it's really the wipers that are the culprit.

Then I thought it was something in the A/C vent that got loose, but that wasn't it.

What it WAS was a 1-inch long piece of thin rubber that was poking out from under the windshield trim in the right upper-hand corner of the windshield. When I drove above a certain speed, it would bang against the windshield and make much more noise than I had thought such a small little thing could do.

I tore the offending piece off (after I couldn't stuff it back under the trim), and the car's been quiet since....

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Janos
1989 560SEL
1993 400E
1994 E320 wagon