Also found this on a BMW forum, it sounds very similar to my issue. (i.e. noise when suspension "drops" or "unloaded" and 'gronk' noise rather than 'clunk' noise)
I'm passing along this advice/experience in the hopes that someone else may benefit from our discovery via long period of trial and error.
I had a recurring thunk/clunk coming from the left rear corner. It made a 'gronk' noise that was rather loud and very annoying. Easily replicated in a quiet parking lot at 10-15mph by first loading and then unloading the left rear corner. Even a small bump would do it.
I suspected worn bushings in the Rogue RSMs; bought a new set and replaced them. Noise abated for around 50 miles then returned. Both Rogue and KW were helpful and responsive on support questions but we could not find the cause.
I Went through much trial and error with my smart and very patient mechanic (Plug for Bruce Barthel, bmwmd.com). We inspected and verified torques on all the bolts of the rear end (frame, suspension, differential, exhaust, etc) but noise persisted. We eventually tracked the problem to the uppermost two coils of the KW spring rubbing together when loading/unloading that corner. The friction had worn through plastic outer coating of the KW spring, allowing the plastic/metal of the coils to bind/rub against one another, especially when that corner got unloaded.
Wrapping the individual coils of that spring with two layers of 3M electrical tape has so far totally eliminated the noise. Not sure yet about the long term reliability on the tape. It will probably last a while here in SoCal. I plan to replace the tape with a synthetic rubber coating product like Plasti-Dip, or some other permanent rubber tubing that I can slip over the coils.
Also, we noticed also that the KW threaded collar was loose enough to wobble slightly on the threaded perch post. This seems odd and not very "German" if you get my drift. We used two passes of plumber’s teflon tape to snug up the fit between the collar and the threaded post.
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2015 Audi Q3 2.0 TFSI FWD
2009 VW Passat 1.8T
2004 Mini Cooper S