This weekend I discovered that the furnace in my old house, when running, creates a vibration , ever so slight, at my enlarger. The bathroom is in between the utility room and the darkroom. I was looking at some projected negatives and I happened to bend over and look through the 10x grain focuser without touching the focuser, my hands were behind my back.....why I did that, idk. I noticed the image in the focuser was vibrating.......and as I was looking at it, the furnace shut off and the image stopped vibrating. I never noticed this before, for whatever reason.......this revelation explains a lot!