i got this amp used, with a loose input jack. i resoldered it, and it was working fine for about a year. out of nowhere it started making a very high pitched squealing noise (NOT feedback) that seems to change pitch depending on how far i turn the volume knob. it happens on both channels. at first it would happen out of nowhere while i was playing, and now i cant play at all. i can faintly hear my guitar...but its pretty much all noise. i notice change in sound if i press down on the cord when plugged in, but i had opened it up and everything looked fine and nothing was loose. PLEASE help me!