The car: 1980 300TD wagon

The problem: VERY loud noise (at least 70db) coming from the left area of the dash (drivers side since its an american make) based on speed. I do not think its based on RPMs but it might be, I revved it in the parking lot earlier and didnt get the noise. Then later driving it would come on about 40mph VERY loud and not get louder as car went faster, but would quiet down when speed was decreased. Its not wind based I do not beleive because it stays loud no matter what direction Im driving and there was no wind out tonite as well. In addition, when the car got all heated up, it would take longer to kick in, about 15 seconds of steady 40+ mph driving for it to come on.

What the HELL is this? I've never heard of something like this before.

Tony