OK, folks -- I did a search on head-fi for this issue and came up empty. Any pointers to it here, if they exist, would be great…
For those of you who have an iPod Video 5th Gen, AND you use Rockbox, there is an issue with persistent low-level sounds in the right channel in Rockbox.
I have confirmed the problem occurs on both the headphone out and the line out. It happens on my iPod Video 5.5G (originally 30GB) and not on my iPod Mini. Both have SSD memory in place of drives in them, and I did some diagnostics on the iPod Video just to be sure (swapped out the CF-to-SD card with a smaller CF-only card, and changed line out cables, to name two). I had never heard of this issue before, and I thought it might be due to the Tarkan board in the iPod Video. But a little bit of internet search turned up this:
The description of the problem is exactly what I am hearing. The rate is about twice a second, which fits the battery check rate. It is a persistent bug and not likely to be fixed. It appears to be isolated to the iPod Video model as it is not there with the iPod Mini OR the iPod Nano 1G, both of which I have and have Rockboxed, and I don't see reports of it on later iPods. It is not on the Known Issues list:
Now, the GOOD news is that it is very very low level. In order to hear it reliably, I have to amp it with a high-gain amp (E12) and turn the amp all the way up. This would make the sound of a normal song deafening. I don't listen to music this way, and I doubt you do either. In fact, one might argue that it is so low level as to be a non-issue for almost all of you.
And of course, it is not there with the stock OS.
For those who have a iPod Video 5th gen that is already running Rockbox, could you test to see if your has this issue? Please be careful with your ears -- you don't need to listen to any file to hear it, just turn up the volume and use sensitive headphones.