this kind of error usually happen when input capacitor/resistor bridge is damaged..another reason can be that your output block capacitors on power amplifier side is broken and since they working as a differential output (mostly for computer amplifiers), what you have is short cut bridging of your signal over one of the broken capacitor in power chain, and thats why you hear 'pumping' no matter is it signal in or not...one of the capacitors releasing over you RC brigdges..it shouldnt be big deal to fix this, so i suggest bring that to any place where they can reapir electronic stuff, and it will work...any Audio service will do that without any problems...if you can control VOLUME, while you have this 'pumping', almost 100% that one of the capacitors in oitput is dead..