What car do you have? From the front to the rear, my 300 has:

Flex disk
Center carrier bearing
U-Joint
Flex disk

I was getting the same rhythmic thrumming you're describing and I went under and wiggled the shaft up and down but I can detect no looseness... Still have the problem.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250