OK, so I drove to a wedding a few weeks ago about 3 hours away, and I'm on the highway. I was probably driving too fast anyway, between 80 and 90, but it was so smooth and nice I really didn't notice. Anyway, everything is fine until I think I hear a thumping sound. It sounds like something is wrong with the exhaust, it sounded like a gasser with a hole in the pipe or something like that, going "doosh, doosh" pretty quickly.

I immediately get a feeling I haven't had since the MG days, the oh-$hit, I'm in the middle of nowhere and my car is making strange sounds and I'm on the way to a wedding feeling. I pull over, and the sound goes away after I drop down in speed. I leave it running and get out, and give it a look over. Nothing wrong. I check the trunk to see if anything was loose and rumbling around. Nope. I shrug and go on.

About 90 minutes later, it does it again when I speed up to around 80 something and stay there for a while. Again, I pull over, and detect nothing. Everything is working fine, no overheat, exhaust isn't loose, nothing. I look under the hood and think maybe the tube that runs over the air cleaner is loose and is vibrating against the cleaner cover, so I make sure its secure.

No problems until one brief moment on the way home, but I kept the speed down. I know this is limited information, and is also strange, but has anyone had a similar experience? Was I in the Bermuda triangle? I'll check with my mechanic when I take it in. I drive it every day around town and haven't had any problems since.

I think my 84 300D did the same thing you are describing a while back. It almost sounded like it was backfiring. It started when my hanger donut.. (was only 1 on there at the time) broke and the muffler drug on the ground for a few miles.

See this thread for more details.

The muffler wasn't collapased on the inside according to a trusted muffler guy. In fact I nearly begged him to find something wrong. So he either didn't want to mess with the ehaust on my car.. or it was fine.

I know this isn't an answer, but your not alone.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.

I think that as the exhaust EXPANDS from the heat of the high speed driving, the MUFFLER is banging at its FRONT EDGE into the frame of the vehicle.....If the exhaust is NOT correctly installed with matching components, then the clearances MAY be TIGHT, resulting in the OCASSIONAL banging.............Ask me HOW I know!!!!!
Spo out