Try moving all output tubes from one channel to another (reverse them, that is). If the noise now comes from the opposite side you will have established a tube (or tubes) as the cause. I don't recall the VT100 tube layout, but move them one row at a time to try and isolate the problem tube, if that's actually the cause. This advice assumes that you have one or more extra tubes, (pretty much a must IMO). If it is a bad tube it can cause others in the same row to run hot, so I would test those too, just to be on the safe side. Any tubes glowing more than usual? Like most manufacturers of tube gear, ARC recommends that you get the tubes from them. For what it's worth, when I've had tubes go I have not had the noise go away as you describe. Anyway, this is an easy way to see if this is the problem. Good luck.
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