This is a '74 Pro Reverb and there are some problems when I have the reverb on. The unwanted noise changes from time to time, but sometimes sound like a typhoon blowing after every note. I have replaced both the preamp tubes that deal with the reverb and there's no change. Also took out the reverb unit and it looks as though all wires/springs are in good order. Is there another common cause of this sort of problem?