I have ProMedia 4.1 which are over 10 years old. Over time, speakers started developing hissing and crackling. I traced the issue to the preamp. It had some gunk/glue on the circuit board, which I cleaned off with electrical-grade isopropyl, and the issue went away. Fast forward a couple/few years, and the noise is back! I checked the preamp, didn't see much accumulation, but cleaned it more anyway. No difference to the noise. The noise comes from all 4 speakers and from the sub When preamp was the source of the problem, I could hear change in crackling when i rotated the control knows. Now knob rotation does not affect the noise opened one of the speakers and checked all connections--all connections appear to be fine Rotated all speakers on the same output and same speaker on all outputs noise is always there one of the channels (rear left) produces significantly higher level of noise than the rest I suppose this means it could have two sources of noise or that there is no central source of noise, and all channels in the amp (or preamp) are impacted, with one of the channels in the amp impacted more than others Unplugged the preamp, and I can hear the subwoofer is still making noise this must mean either that the sub has two sources of noise, and the issue could still be in the preamp or the preamp is not the source of the noise Opened the amp, checked all the connections in the amp and from the amp to the sub--everything is nice and snug Examined the circuit boards in the amp--no signs of component wear and foreign matter I am at a bit of a loss as to what to do next. Has anyone experienced similar and/or has any suggestions? Does anyone have experience testing individual components of the amp? Any other basic tests I missed? Thanks!