i've seen this same issue if your amp is grounded to the chassie and its close to where the fuel pump grounds to the chassie.

usually its hard to tell where the fuel pump is grounded as its grounded from underneath, you can tell if this issue the issue , becouse the sound will be there when the car is running, and not there when just on accessory power.

 

yes, you want to avoid ground loops like the plague...


regards, Mark

 

I am guessing your speakers are amped?

I am willing to bet your amp gains are too high and you aren't using the full output of the decks preout voltage. Reset up you system and try it or take it to a shop and have them do it for you..

 

check all your grounds.
try moving your grounds to get a good ground