it wouldn't suprise me if it was cell phone or EMI noise causing it, my cell phone can put noise though my TV, my computer's audio line outs, and my previous recorder's analog in (i river h-120)
if you have heard it before you can eventually pick up a pattern that it makes each time a call comes in
my unsupported 2 pennies, go test it out, id love to hear about it... ill try to get my JB3 to do it without a UA-5,
or if someone wants to give me a UA-5, id try it too then