The ground loop was being caused by the Video Processor after all. It must have something to do with the AC/DC converter the VP uses. I used a ground isolation adapter on the VP and the thump went away. The humming I was getting with the addition of an amp went away too.

Rob, quite a few of us have done what you did. It’s the mystery thump. I know of at least two others besides myself, and now you, that have had to cycle the power to kill the thumping ghost. This last situation that I have been fighting is different.