i got the same thing, when its loud enough i cant hear it. lol. thats my current fix. try cleaning your earth point? i also have rear speakers amped. lets know if you find a solution.
if the sound is only evident when the vehicle is running, usually its engine noise. common fixes are better earth cable for battery, better route for the power cables, separate the rcas and power leads for amps as much as possible, install noise suppressors "ground loop isolator" on the speaker rca's at the front or the rear of the run, improve amp earth points by sanding the mounting area back for a clean contact. use higher grade cables etc.