Hello, I really did try to find a similar question/problem. My '91 240, w/ 114K miles, has developed a nasty droning, rattling sound while at speed. It sounds like someone put a few small marbles, or a ball point pen down the heater ducts. It's a "plasticky" sound, if that helps. I can sort of push and put pressure on various points while driving, but nothing makes a difference.

It seems to be getting worse, but before I start tearing the dash to pieces, I'd like to hear from anyone who has tracked down this kind of problem. Really, it sounds like something is rubbing, or pulsing , and there's no difference if the heater or blower motor is on, or not. Good thing that my stereo system goes to "11", but not a long term solution!!