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?