Originally Posted by Technic
It could be possible one crossover is defective, less probably both.
I was looking to remove the front speakers variable and concentrate in the possibility of the RCA signal as the source of the problem. In other words, if the noise was still there with the OEM speakers then there must be something wrong in the harness RCA contacts -assuming the same problem with two different amps and two different RCA extension cables.
If the noise goes away with the OEM speakers then the problem must be the crossover circuit.
Unplug the tweeters at least and see how this affects the noise.
I see where you are going with this. I can't try that right now, but may have sometime to take the door apart next weekend.
Meanwhile, I just replaced the HU with an ipod and Y RCA cable, the hissing noise is gone when not playing or paused. I think the problem is with the connection and feedback from the HU.