This is interesting. I've also got an SP-SPSB-4, PCM-A10, and SP-CMC-2 mics and have never noticed such a sound. I just checked and NFC was on, I just turned it off because I'm almost positive I don't need it turned on. Can you post a short audio example, please?
EDIT: Wait, after re-reading, why are you using MIC-IN and not LINE-IN with the battery box?