that would be what is commonly know as alternator noise it has a lot to dew with grounds try pulling out the deck and making sure the harness is not loose if thats ok and u still have the noise make sure the batt bolts are tight and if that dont work u can go to best buy or a car audio store and get a ground loop noise reducer (make this your last resort can be difficult to put it on a stock deck) and 1 more thing if its only 1 speaker check the wires on it make sure there on snug