It's regarding my 87 300D. Today I started hearing a single thump from the rear every time I quickly release the throttle while driving at above 2000 rpm or so. If I release it slowly or at lower rpm I don't hear it. What could it be?

By the way I had new tires installed today and I started hearing this sound shortly afterward, but not immediately. I'm thinking this is a coincidence or could it be related? Lug nuts are all tight.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL