Ok....first things first.....unplug your RCA's from the amp, and listen. If the noise is still there, that means it's obviously a power line noise, or your amp is ****ed. If this is the case, try moving your ground, and make sure the screws that you mounted the amp with aren't touching other body metal because this will cause a ground loop. If however the noise goes away, try another set of RCA's ran quickly over your seat to the deck, to see if the noise goes away......If not, its probably a deck ground, or possibly the deck itself

 

what are your gains set at ?