Two of the 5 amps I've owned do this. (B-52 ATX-100 and my Triple Rec). While they're sitting in standby mode, and also right after I turn off the amp, they make a chirping type of noise, like a cricket. Is this normal? None of the lower wattage heads I've had do this -- only the big boys. Is this a sign of a dying tube or something? Everything sounds fine.
May just be the glass in the valves expanding on the warm up and contracting on the cool down?