disconect the wire from the HU to make sure the noise is not coming from there,
if the noise stops when you disconect that RCA from the HU, then the problem is in the RCA output of the HU,
if the sound continues, the while the RCA cables are still disconected at the HU. disconect them RCA cables from the amp, if noise goes away then the problem is in the RCA wire it self.
if the noise persist. then the amp has the noise internaly.
since you already whent thrug some of those steps and ruled out the HU and only one side is creating the noise.
then for testing purposes.
move the good RCA on the amp side, to the side where the noise comes from, if the noise starts again then, you have a problem on the RCA input of the amp.
if there is no noise. then move the good RCA line on the head unit to the one that may be making noise. if it starts making noise then the problem is on that one HU RCA output. if there is no noise then the problem was the RCA cable.
check the manual on the amp some stereo amps require that you disconect one side of the rcas. when runing the amp in mono. i dont know what the case may be.