I had the exact same problem with my '90 300te. The problem began when I replaced the high pressure hydraulic hose with one made at a hose specialist. This is one of those places that custom make any type of hose for race cars and such. They removed and reused my fittings onto a new length of hose. The new hose had a slightly smaller diameter. The first time after I installed I got a very fast vibration sound that changed a bit with increased RPM's, but really changed whenever back pressure was introduced into the system by hitting a bump or dip in the road. The high pitched vibration sound seemed to be stronger on the passenger side. The best way I can describe the sound was that it was like the noise you got as a kid when you clipped a playing card to your bike and it made noise when hitting against the spokes, only faster.

I confirmed my suspicions by grabbing the line at at various points with the engine running. I could feel the vibration and it was the strongest at the end towards the rear of the car. I took the hose back to the shop and they re-welded a larger diameter fitting since they suspected that the fluid was being restricted in some way when it was being needed to support the rear suspension. That hose ended up having the same problem.

I read a recommendation from the forum to only use an original MB part for the high pressure hose. I changed out the hose with one from the dealer. Problem solved.

In retrospect, I have concluded that there is a reason MB calls this an Expansion Hose. Just a guess, but I think that when the system gets back-pressure, the hose actually expands. If you replace with a less flexible one, vibration results.