I have a BHK signature preamp, about four months old. Recently I have noticed a high pitched whine from the unit that is annoyingly audible from my listening position (about six feet away). I’m pretty sure it was not making this noise when I first received it. Just to be clear this is not a noise through the speakers; it is a noise generated by something in the preamp. It stays at the same level regardless of where the volume is set, but it disappears immediately when I put the unit into standby by pressing the button on the front. If the unit is warm, the noise comes back right away when I turn it back on. It is there while the preamp goes through its 45 second warm-up cycle and gets a bit louder when the warm-up cycle is over and the unit clicks on. If I turn it off using the switch on the back, the noise disappears and does not return until after the 90 second warm-up cycle is over.
Any ideas what could be causing this? Is it normal behavior and I just didn’t notice it until recently? Could it be tube-related?