This is in my experience a fairly rare type of problem. The radio has an automatic gain control that reacts to the signal strength of a station. So with a radio operating correctly, the removal of the antenna pretty much removes any source of static. Sometimes with the antenna out you can here some power supply noise but this is irrelevant. If you wanted to pursue some trouble shooting I would suggest the tube removal sequence that I described. That would probably isolate the stage pretty quickly. If you a volt meter I could probably walk you through further steps. It does sound very similar to the problem I had and as I said it is a rather unusual one. I swapped out my radio for my spare maybe four times over a few years only to have it behave on the bench. In the end it malfunctioned on the bench and I was able to diagnose the problem.
