also check the wheel hub bearing, it might need to be repacked (replacing or adding up new grease) or retighten, had that problem before on mine...

Mystery noise source identified and fixed.

Isolated noise to shock absorbers with my stethescope but still didn't know what was causing noise or how to fix it. So I took it to a friend who owns an MB shop in town for him to diagnosis. He had not heard a wagon making the sound mine was making when you bounced the rear up and down manually, without the problem being one of the ones I tested for.

He also thought it was the shocks so he manually filled and dumped the fluid from the control valve for the self leveling rear suspension with a couple of us sitting on the rear of the wagon with the tailgate open.

That fixed it. The problem was the shocks were not sufficiently full of hydraulic fluid. We don't know if the noise was air or what since the noise was present on both rebound and compression.