high pitched noise from system

Here is what I am running (relevant to this issue) from newest component installed to oldest: an Alpine PDX 4.100 4 channel ampifier, a Clarion MAX685BT headunit, a pair of Alpine SPX-17REF in the doors and a pair of Alpine SPR-57c in the rear. The car is a 1996 Lincoln Mark VIII and the headunit was installed about 2 months ago, and the Amp about a 5 to 6 weeks ago. There is also a pair of subs run through aPDX 1.600 amp, but I don't think that is relevant to this issue.I recently started noticing a slight high pitch noise coming from the system. It didn't stay all the time but came on and off; intermitently. It did it on any input (DVD/CD, radio or Ipod)and it didn't matter if the car was running or not. It did not increase in pitch or intensity as the engine was reved, and it was present at all volume levels except zero.Thinking it might be the gains, I turned them all the way down and then listened for it. I couldn't hear it at any volume, though that could be because the signal wasn't boosted enough. I started adjusting the gains, and at even a very minor adjustment, the noise started coming back intermitently. It seemed more pronounced in the rear when adjusting the gains; as soon as the gain was moved it started making the noise. However, since the noise is so faint, it was hard to really tell.I am never hard on the system, and used a 1000Hz test tones to initially set the gains, as well as readjust them and check for the noise.I have checked all the connections on the headunit and the amps, and they are all fine. Any idea what it could be? What should be my next step(s) for troubleshooting this?