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.