Anyone know what causes this?

[Phil Nelson]

My Hallicrafters T-67 had Barkhausen interference. The cure was to substitute a horizontal output tube that didn't oscillate.

It does seem odd that you'd have four sets with that identical problem, but what do I know.

One of my old TV books has a good discussion of Barkhausen. Besides subbing the tube, they suggest reducing the drive, or even putting a small circular magnet around the output tube and rotating it until the oscillation is suppressed.

I don't know if what you have is Barkhausen effect, but those are all pretty easy things to try.