It's a Pioneer. Do you have a DMM and know how to measure resistance?
If so check the resistance from the HU's chassis to the HU's RCA ground (out side connector on the RCA jack). It should be vary low, for a Pioneer it should be near 0 ohms. If not there is now or was a bad ground in the system that damage the HU. But it can be corrected. The other thing to try would be to, with the system one and noise present, take a small screw drive and press it to the chassis of the HU then at the same time hit the RCA ground. You may have to put the RCA connector out a little to get at the RCA ground. Did the noise stop?