Ground issue. You're gonna need to do a little trial and error tracking it down. It could be the interconnect. Or it could be the RCA connection itself, or somewhere inside the amp. It could even be things plugged into different outlets. Or maybe even something else.
Clean the RCAs real good. Both ends, male and female. Probably not this but since nobody ever cleans anything you need to do it anyway so start with this.
Clean the RCAs real good. Both ends, male and female. Probably not this but since nobody ever cleans anything you need to do it anyway so start with this.