Just connected my rear speakers, an i hear a faint engine whine, its not loud, but when idling its noticeable, an annoying to say the least.

This only happened when i plugged in the rear speaker RCA cable, yet when i take out the rear rca cables, the fronts splits or speakers have no engine noise what so eve.

channels 1 & 2 - 6" splits

channels 3 & 4 - 6 by 9

So im not sure if its the actual RCA cable itself or some other....

Ive re checked all the wiring, and connections, nothing is loose, so it doesnt add up :S

RCA and speakers run on the RHS, power LHS

Currently running a 4 channel amp an mono.....

what im a doing wrong here?