My 500SEL once did the same. My local, trusty agent told me he had seen a few installations with an internal break up of the pressure pipe from the pump to the box in the past and replacing the pipe had fixed a few of them. In my case, a relatively inexpensive rebuild of the pump did the trick. You can see if the belts are slipping if you drip water on the pulleys with the wheel hard over and the engine running, the screaching should stop for a while until the water dries out. Just my 2 dirhams worth !