Pump impeller went crooked, apparently both from wear and moving the unit. Would not then stay in place.

Disconnected hoses (2) from pump housing. Disconnected pump housing from motor (3 allen screws, not too acessible). Unscrew spray arm from pump housing (this is done by hand; no tools as the "nut" is about 3" diameter and made of plastic) Open up the pump casing and replace impeller and the bearings on each side. The thing that takes some time is to mop the water that comes off at almost each step. There is probably an intelligent way to drain everything first. I did not order the large seal (kind of a large rubber donut, larger than a donut) between the arm and the pump and I shoiuld have as it was difficult to keep the old one from not leaking: Since by hand you cannot tighten the large nut very hard and tools could damage or break it. Applying upward pressure on the pump while tightening did it. (The pump and motor are all mounted on flexible mounts, like the heavy hoses connecting to the pump housing, so that there are no vibration.)