yung sound na yun ser malamang sa ground loop yun.. ideally dapat single point ang grounding ng amp at tsaka head unit.. can you verify if the noise is present or not with the following conditions?
1) power amp on, head unit at minimum volume.
2) power amp on, head unit off (you might need no override the remote input of the amp to force it on)
3) power amp on, inputs disconnected from head unit
4) power amp on, inputs terminated (shorted or terminated with resistor)
btw, what brand is your head unit?