There is circuitry built into the the HU (sometimes the noise filter is an external black blox on the power wire of the HU) and amp for noise isolation....this can sometimes fail...it can be bad and everything else could still work great...unless its done it since the install my guess is either something failed or one of your grounds is failing...or perhaps you have a damaged plug wire?

 

as in spark plug wire? no i have new msd 8.5 mm's


it could be the headunit since i got that 5 years ago.

 

yeah I meant plug wire...they are #1 source of radio interference...
I would still double check all your grounds....some rca cables even had an additional ground wire at each end to help shield against this...I used them in my jeep.

 

I did'nt read through all the responses here so i dont know if this was stated yet, alternator whine.... your rca cable is to close to your power cable.