RFI or Capacitor issue?

We have an Audion Silver Night MKII, and it powers up silently. After it warms up a bit, I periodically get some light hissing in the left speaker, with an occasional high frequency noise that will rise in pitch for about 2-3 seconds. When I swapped the speaker cables, it moved to the right speaker. I've tried different tubes, and there is no difference. I've swapped tubes L/R, and there is no difference. I've got an appointment to take the amp into the distributor and have his tech look at it. He thinks that a capacitor is the likely culprit. But I want to make sure that I'm bothering him with a legitimate problem. I've not heard RFI or EMF noise that I know of, and wonder if that whining electrical sound could be interference from my wife's computer downstairs, or something of the sort.
Thank you all for your help.
Howard