hum

I’ve got a 1500 watt amp and 2 kicker 12” subs that play a loud hum when I turn the car off. This note plays during the time between the engine shutting off and the amp getting cut off. ALSO: I noticed that if I unplug thr RCA from the amp, this does not happen. What might be the issue?

I've been reseaching about why subs hum, and I've tried pretty much everything I've read, except things like LOC and Ground-loop isolator. I traded someone in my family cash for an audio system out of a Ford Ranger.(Maybe it was damaged or dropped is the problem???) I put it in my 98 Sable, so...