weird fluttering sound, '83 500SL Euro

Anyone ever have anything like this? It's kind of a quiet-ish fluttering sound, sort of like if you took a dry sink sponge and attached it to a bicycle frame where the spokes would hit it (you know, like little kids used to do with playing cards to make bicycles sound like mototcycles). The sound and volume are about like an old Volkswagon bug putt-putting a few lanes away from you on the freeway. (In fact, that's what I thought it was the first time I heard it.) It only does it at about 30 MPH and up. Never when stopped, and you can't tell which end of the car it's coming from. The volume, speed and pitch never vary, regardless of road speed or engine speed, temperature, etc. It doesn't even change when you put it into neutral while driving. It almost sounds like a vacuum pump, beating maybe 3 or 4 times a second and lasting maybe 20 or 30 seconds at a time. It's intermittant--usually only once or twice a day. Been doing it maybe 3,000 miles. Do relay switches ever do anything like that when they go bad? Is there a vacuum pump that might be leaking? Any thoughts appreciated, guys. I'm about to go nuts!

Figured this one out finally. I'd put an aftermarket radiator fan shroud in. I guess the plastic is too flexible--at certain speeds, the wind flow was causing it to flex, causing a fluttering or beating sound. I put in a little brace to correct. Just in case anyone else has the problem.