Have an AV receiver (Yamaha one for surround sound) with 5 speakers and a sub. The sub started making some really weird noises, as if somebody was knocking on the door randomly. Did a bit of googling that seems to think it's the receiver / amp rather than the sub, have ordered a new cable to rule that but don't fancy buying a new amp without properly testing. Havent got another sub to rule that out either. When the sub is on normally the sound doesn't seem to match up, it's just random. Edit: the power light doesnt seem to be on the sub, when I set it to off the light goes red. If I set it to on then pull the power cable it goes bright green for a second then off. Anybody know any decent repair shops that could have a look?