I bought a Rockford Fosgate 400 watt 4 channel amp off a coworker and I did not use it for about a year but it started humming through the speakers a month after I installed it,it was not motor noise and would come and go and would start doing it after it warmed up. No more used amps from now on.

 

Yes. Capacitors can dry out and then you've got a dirty power supply feeding dirty amplifier stages.