So the sound doesn't seem to come out when it's warmer - it's more of a cold weather thing, and seems to happen over more RPMs now (still low-revs, or perhaps the sound is just drowned out at higher revs?).
Tried the bar of soap thing. Didn't make a difference. I suspect it's the tensioner, be it just the pulley or the whole assembly, I don't know.
I'd change out the pulley and a belt, but I'm not willing to do the tensioner. Any idea what the cost would be to replace "professionally"?