My wife and I have a noise from our '81 240D (automatic) and we could use some educated guesses.

The noise seems to come from the middle/rear of the car, I'll say near the middle as it can be heard from either side while we're driving in the beast. The sound is a kind of clunk-clunk, or thunk-thunk, that increases/decreases with the speed of the vehicle.

The sound has sounded on three occasions, each time after the car has been driven for at least an hour on the freeway, bombing along at 75+ , but only on those three occasions, not on short freeway drives.

This weekend, head up to Ventura, car spinning like a top, an hour later pull off the freeway, wait at stoplight, pull away from stoplight - thunk-thunk-thunNK-THUNK...!!??

Stop car, look underneath, nothing dragging or hanging. Wife says: this is what it did a month ago when I came back from Fresno! We walk away from car for five minutes, come back. Start car again, still making noise (quietly as we're moving slowly into a gas station parking lot). I get out so she can drive a short distance while I try to pinpoint where the heck the noise is from. After I get out - no noise. Rest of weekend, local driving and a 20 minute freeway blast, no noise. Car runs just fine.

Monday come home from Ventura. An hour later, pull off freeway, stop at light, pull away: thunk-thunk-thunNK-THUNK...!!?? She hops out and runs alongside, thinks its coming from the middle/rear of the car.

She has yet to drive the car today, but I won't be surprised if the noise is gone again.

To recap:

Noise from rear/middle of car (our best guess)
Regular beat - i.e., not a lug nut in a hub cap (happened to the Olds once)
No obvious sensation of anything hitting the bottom of the car
Happened a month ago after long sustained freeway blast - went away almost immediately.
Happened this weekend twice - after hour long freeway blasts - went away.

Did not happen after 20 minute freeway blast.
Not an object poking out of the tires.

I apologize for apparent laziness, we live on an incline with no garage or jack stands. I slid under the car looking for sheered metal, oily stains, anything that would look suspicious to a noob. Nothing but dirty and solid (looking) hunks of car under there.

We have a shop with a good reputation in the area who I probably should be giving this info to, but thought to run it past the members. Anyone care to take a guess?

__________________
1981 240D Automatic - "Putty" 222K mi.
2008 Honda Fit Sport - wife's appliance

Sold due (in part) to now owning a child:
1969 Oldsmobile Cutlass - "The Baby" 160K mi.
2006 Ducati Multistrada 620 - 7.5K mi.)