Likely a ground issue. Make sure the amp, head unit, alternator and battery all have a good solid ground to the chassis with good size power cable. Remember that current flows from negative to positive through BOTH cables so they should be of equal size and current capability. In general the RCA cables should be routed as far away from the power cables as possible. When I was installing I routed the audio cables down one side of the vehicle then the power cables down the other side, sometimes it is good to run the audio cables down the center of the vehicle.
Sometimes the filtering in the alternator can go bad as well.
You can try unplugging the RCA cables from the amp and see if the noise goes away. If it does the noise is being radiated into the cables or coming through the head unit. If it stays then there is a problem with the amp, the power/ground wiring or the battery/alternator. Then unplug the RCAs from the head unit and listen again. If the noise is now gone the problem could be in the head unit or head unit wiring. If it is still there it is likely being radiated into the RCA cables OR there could be a ground issue in the cables from a bad connector end or something like that.
I hope this helps...
