Could be on the back of the deck, or it might be an engine whine going through your speakers... All the speakers are amped im assuming?? If so make sure the battery wire is running down the speerate side of the car, and they don't come close to eachother..
ur taking to much juice from ur battery and its makin ur alternator hum in ur speakers, u may want to try to run a seperate battery just for ur amp,subs etc. and stuff, unless u already have done that
I'm guessing its a new deck, so check your gains on your amps. I had that problem when I installed my stuff. Make sure your gain matches your preamp out voltage.