Wailing noise from center of dash

[Flop]

Hi everyone,

I'll get to my question in a second, but first a word of thanks.

A couple weeks ago, I decided that I wanted to buy an old diesel Mercedes. I began researching them on the web because I'd never even been in a Mercedes before. I didn't know what the different model numbers meant or what I should be looking for. During my search, I came across this forum and it has been fantastic. So thanks to all of you for the knowledge you've provided me - before I even have a Mercedes. Not only that, but reading these posts took me from considering an old Mercedes to I-will-buy-nothing-other-than-an-old-Mercedes status.

Ok, now for the question. I think I have found my car. It's a 1980 240d and is in great shape. I test drove it this morning and it ran great, but it makes a loud whistling/shrieking/wailing-like-a-banshee noise. It only makes the noise when the car is moving and it's doesn't seem to be tied to the engine. If I rev the engine or put it in neutral as I'm moving along, the noise does not change. When at a stop and idling, there is no noise. It sounds like it's coming from the center of the dash, possibly through the center vents. I think I closed them by sliding that mysterious lever, but it had no effect on the noise. I tried all combinations of the climate controls and it had no effect on the noise. Even if the fan was completely off, the noise was still present. My best guess is that it is wind noise somehow making it into the car. But it's extremely loud. It varies in pitch and stops every once in awhile (while moving). I've scanned this forum for over an hour and can't find any other mention of this particular noise. Any thoughts are greatly appreciated.

Thanks,
Flop