My 81' 240d will make a whistle from about 2000rpm to about 3000rpm, you can't hear it at all with the windows up, but it's fairly loud from outside the car and with the windows down.

I can't figure out what it is, I haven't noticed any loose vacuum tubes, but it sounds like a vacuum leak.

Any ideas?

__________________
'95 Geo Metro 3-cyl 5-speed
'97 Geo Metro 3-cyl 5-speed x2
'81 Mercedes 240D 4-speed manual RIP