What does

For years I have been reading user comments on this forum about dirty power and how it can be corrected or minimized with power conditioners and dedicated circuits. I have recently been plagued by an annoying hissing sound that appears in only one of my two speakers at a given time and sounds an awful lot like radio station static. Sometimes it in the right channel, sometimes it is in the left channel. It is not there all of the time, but shows up only occasionally. The sound seems to last anywhere from a few minutes to the better part of an hour or so before it simply disappears. Dirty power? Time for some dedicated circuits?
I would think that if you had a dirty power issue that it would be in both speakers unless you are running mono blocks on separate circuits. Are you running tubes in any part of your system. Could it be a tube going bad?

If you list your complete system, we could probably better answer your question.
Thanks Eldarado...I would have suspected the same thing, both speakers, but no, one at a time. I am running a low-power tube amp, one output tube each side. Switching tubes simultaneously has no effect on the speaker making the noise.
Wow this one is tricky. I still think it would be in both speakers at the same time if it was the power. Could it be RFI with your cables. Does it happen when certain appliances are running. Like the microwave? Well I am now just grabbing for air.

I am curious to see what the issue is.
Thanks again, El...quite puzzling. Unrelated to appliances, as problem sometimes occurs when nothing is running.
You say it disappears, but it always returns later. Could it possibly be a leaking capacitor in amp or preamp ? I'm no expert on electronics, but you might run a check on those caps.
Maybe try another outlet or even better yet another outlet on a different circuit. Any flourescent or dimmable lighting on the suspect circuit? Maybe a bad ballast somewhere??