I'm said customer.

Where is the filter? I'll check it.

And just as a quick tidbit...I don't think it thumps whilst driving; only idle. I *think*.

If you run the arm through 'F' and 'L', the thumping stops in those positions. It also seems to stop (as to be expected from that) with a heavier load in the back of the wagon.

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2004 E320 Wagon W211
2000 CLK430 Convertible W208
1985 300SD W126
1983 300D W123