[codefox1]

I've seen (heard) this once in a while. Sometimes it is just that the mounting bolts for the transformer are loose, and , of course transformers do hum. So this can be fixed by tightening them up.

Other times, expecially if the transformer has overheated, the wax or tar melts away, eliminating the damping factor, so the whole assembly hums. The fix is to remove the bells, and tighten everything up with some filler and padding. There are as many ways to do this as there are members of this forum. More exotic methods are removing the transformer entirely, soaking the core and windings in varnish for a long time, and baking it for several hours, and letting it sit for a couple of weeks.

I've never done that, but I have glued the whole thing up with gobs of Elmer's Glue and set the core and windings atop the oil burner for about a week. Others dab on some epoxy to keep the winding stationary on the core.

The least and fastest, and most reversable method is to put some foam rubber in the space between the windings and the bells, such that when tightening up the bolts, the windings will not be able to move.

Having said that, I assume you have ruled out any electrical reason for the hum, as in bad filter caps, tubes with cathode to heater shorts, gassy output tube, etc... Let us know more!