This may seem like out in left field, but I had a similar thumping sound that increased with vehicle speed. Changed flex discs, but thumping still there. It was the inner sidewall of the rear tires that was coming apart. Outer sidewalls looked perfect, but for some reason the inner sidewalls were bad. Perhaps its worth a check.

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ARVY

1999 E430
1999 C230 Kompressor
1992 300E (write-off)
1984 190D 2.2 (sold)