You have a ground loop. HU is grounded up front, amp in the rear, different ground potentials, and the power ground is traveling through your rca cables. Ground your head unit in the same place as your amplifier. You can install a ground loop isolator, but it will reduce you rca preamp signal strength.
__________________
-2000 Toyota 4Runner Limited 4WD Supercharged (silver/black, heavily modded, Team JBL)
-1991 Chevrolet Corvette ZR-1 (red/gray, stock)
-2004 Sentra SER (yellow, stock)