subwoofer noise, but no whine

I just installed an 8 inch Kicker sub in my 03 Denali.  The HU is Kenwood DDX 8017, and the amp is Hifonics.  The complaint is that the sub makes a low humming noise at key on, and at idle.  The noise doesn't get worse as RPM go up, but you can definetly hear it at idle.  I made sure that my ground to my amp was clean, and changed the ground to the HU.  The noise occurs using either channel on the amp ( I tried both).   I also tried hooking up different RCA's  (from stinger), but the noise persisted.  I have kind of hit a wall, any help would be much appreciated.

thanks,

todd