i have a koss KS4102 system also and mine does the same thing. its done it for almost 2 yrs now. ive taken it apart and noticed that the sound is coming from under the heat sink. im not an expert at home audio, but my best guess is its either the amplifier going out, or some kind of processor. but i do know from past experience that its probably going to be cheaper to buy a new receiver, rather than paying a pro to fix it, if it can be.