Still need to do some more investigation on this one btw. I also think I'm slightly mixed up on my rockbox versions (though no matter which version I try the end result was the same!). According to rockbox-info.txt, I'm using version "1cada1f-150911".
One thing I've noticed though I'm not sure if it is just a fluke or not is that it seems
like the interference might be mitigated somewhat by what sort of headphones you attach to the device. This really shouldn't matter but maybe it's some sort of weird impedence mismatch/problem or prehaps some headphones cause the device to "pick itself up" more than others - I have no idea.Here
are 3 headphones that I used with my clip plus while recording the FM radio.
Top left is a Shure SRH440 (expensive!) headphones. Top right those are Sennheister EH1430 headphones (from the late 90s, and with a very long cable). At the bottom, a cheapo "sony walkman" headphones, model MDR-027.
I tried recording the FM radio with all three. I'm still really investigating here, but for some reason if I used the shure headphones I got the exact same problem above. The sennheister and cheapo sony headphones were *much* quieter - in fact I could hardly hear the noise.
I still have to try out different sample rates and other rockbox versions.