The 928 engine rests on two hydraulic mounts. These tend to collaps after time due to weight and engine vibrations.This causes the engine to sit low and vibrations transfer into the body of the car causing unpleasant rattle and sensations. The collapsed mounts need replacing. Porsche sells the hydraulic mounts at 395 euro's each (!) and you need 2 of them. I offer Anchor replacement hydraulic mounts at a fraction of the cost. I sell them only as a set, so 2 units are in a set. Fitting can be done by lowering the cross member and supporting the engine under the oil pan or from above. It is not a very difficult job. Instructions are on my web.
More information on this page: http://jenniskens.livedsl.nl/Technical/Tips/3/MyTip304.htm