... and then quiets down when you start pushing the car a little harder?
Well, here's the culprit...
Check the rear shocks. This little set screw that keeps the springs tightened up against the shock body can back out, and the adjustment ring rotates down.
This allows the spring to rattle around against the shock body.
Simple fix - snug up the ring, locktite on the setscrew, tighten with an allen wrench.
It's a funny design. There is only a single setscrew locking the adjustment, but there seem to be a lot of holes (most likely for the special adjuster wrench).
Here's where it goes.
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