Amp gurus.... I have a issue with what I’d call a feedback hiss or noise coming out of my fairly new Princeton Amp, about 3 months old. Is a low oscillating background noise kind of high pitched. It has shown up recently and comes on after the the amp warms up. At first I thought it was coming from another room, running water at a sink or something. I've turned the volume off and to max, the noise stays that same level. I've done the same with all the other controls and had the same result with the noise staying at the same level. It's not a hum, I've put my guitar next to the amp and do get a slight hum. I've played my custom champ and no similar issue. I'm not a electrical engineer, any one had a similar issue? I live in a large metro area, close to a flight path at Dulles Airport. Any thoughts?