This is exactly what I had, since you mention tapping makes it go away
you might have a dry/cold solder joint on one of the vertical IC pins.
in My set it was an LA7838 or something similar. Just hit eag of the
pin with a soldering iron and you might be good to go, one additional
test is to plug the set in and while running tap the heatsink that the
vertical IC is bolted to with a wooden dowel, if the picture exhibits
the behaviour you mentioned or it goes away you know it's a cold solder
joint