Whirring, whistling sound from left speaker

When I turn on my preamp and amps, but with the preamp in mute mode, I hear a whirring, almost whistling-like sound from the left speaker for about 1 to 2 minutes. This happens whether I have a power conditioner connected to the amps or not. I have independant circuits in the house for the system. Both the preamp and monoblock amps are tubed. Any ideas why this is happening and whether it is something I should worry about?
In my experience, this is a sign that a tube is failing and about to die. If it's a multi-tube preamp, for instance, you have to remove and replace until the troublemaker tube is identified. Same for amp if the problem is not with the preamp.

Good luck,
Yah many times..It does sound like a tube at the end of its life...Sometimes it sounds like bacon in the frying pan!Try fliping left and right tubes and see if the noise travels with the tubes..More than likely you will be buying tubes soon............
Sure sounds like a bad tube to me too. Do what Thorman said. Swap the left channel tubes with the right channel tubes and see if the noise changes sides.


Thasks to all of you for your helpful responses.
Well, I replaced the tubes but the sound still occurs. Any other ideas?
Hi : The only other thing I can think of offhand is your incoming power..I sometimes get some unpleasant noise usuaully during are heavy electrical usuage ..Its usually during real cold weather or very hot ( many air conditioners working )muggy days...For me its garbage coming in from the electrical comapany about 50% of the time..I have learned to just live with it.........