Mute each type of sound one at a time until you find out where it's coming from. I think it's part of the background music.

I did that when I couldn't work out what the hell the annoying sound was, it's not part of the musical soundtrack, it's an effect, when I turned effects down the noise went. Haven't heard it for a while and I can't recall if I had restoration spells out (I doubt I did - silly thing to do running around with healing spells on hand) but I do know it's not a damn Nirnroot, it's way more subtle than that.

Damn you!!! Now i have that song in my head again xD