I started getting engine noise through my back speakers. I tapped the amp with my small plastice mallet and it stopped. Upon checking, I would wiggle the small plug on the side and start and stop the hum. I figured the plug was lose or broken, so I took it out and resoldered the two connectors, insulated them and plugged it back in. Lasted a week and the hum came back. I found another amp at a wrecking yard and installed it. So far, works better than the old one ever did. Cost was $36.49 and that was less than an electronic shop would charge to look at it.......Did the Happy Dance.