Its alternator whine and can be there due to a load of reasons. What we can try is the elimintation method
At what point is ur amp gain? try reducing it to 15-20% and then chk for noise, and if it is gone , tune further from ur headunit and dont increase it. Prevolt mismatch can cause this aswell 
1- swap the Rcas to someother ones (will rectify if rca is the issue)
2- swap the amp temporarily with another ( will rectify if amp has ground blown)
If both dont solve it
Headunit is the culprit.
Chk upto this and tell me , HU ka masla hua tau ill tell u a easy way out 