Audio problem: Noise channel...scratchy?

[TylerL]

I had left my AV modded Famicom on overnight (I really wish the Everdrive could do savestates...), and when I returned, music sounded very off.
The best I can describe it is that the noise channel in the audio output sounds "scratchy".
Here's a recording of a familiar theme: https://dl.dropboxusercontent.com/u/56481592/marioweird.mp3

This persists in many different games, and on native carts as well.  

Any thoughts? Seems strange that it only affects a single audio channel...

[aha2940]

The famicom/NES have two pins for audio, which come from the CPU. Pins 1 and 2 IIRC. Maybe you have problems with the pin that produces the white noise signal. Does the problem persist after whutting down the famicom, waiting some time and turning it on again?

Regards.

[TylerL]

I had cracked the Famicom open to have a look last night and didn't see (or smell) anything out of line.
In the course of my inspection, it had been turned off and unplugged numerous times over an hour or so. I had given up hope and chalked it up to being an idiot and shorting something out during my last modding session.

...then, after leaving it unplugged for 8 hours overnight, I turned it on this morning and it sounded almost fine!
No scratchyness. The noise channel seemed a bit more pronounced, but it could just be that I'm more sensitive to it now.
I'd like to compare it to a "reference" recording later today just in case...
Post Merge: May 09, 2013, 06:47:48 pm
So, for MOST playthroughs, the noise channel is back to normal.
Occasionally though, the scratchiness comes back. Sometimes a simple reset is all that's needed to recover proper sound.

Does anyone have any insight to how the noise channel operates? It's based in some capacity on randomness, isn't it? What "produces" that randomness?
...or am I looking in the wrong place?