Is there a console command or some other way to find out what is "attacking" me? There is a small area in Caldera near the Mage guild where the combat music ALWAYS plays. I looked everywhere but cant find the hostile enemy. Is there a command to list "currently engaged enemies" or something along those lines?

If you're a mage, cast detect creature and then check the map - it will show your enemies marked in red.
If you're not a mage and play on PC, you can probably use console to get this this spell and use it - just to check what's attacking you and where it is located.

Did you also check all the way up and all the way down? Caldera... There's no water there? Usually these music changers are slaughterfish. Might be a cliff racer too that can't get down, or some random beast that followed you for awhile but got stuck in the mountain or something. :/

This is correct. I once spent fifteen minutes looking for something that prevented my character from resting, only to discover a kwama forager that was wedged some distance away between two boulders.

I'm betting it's a cliff racer. There's one that spawns above and behind the mages guild there, and it very slowly attacks, but you can't see it until it comes over the building. It may actually be stuck on the building; cliff racers often get stuck on things.

Solution is to go away from the area (several cells distant) for at least 3 days, and the random creatures will respawn. If that doesn't clear it, then you've got an angry NPC with a grudge against you stuck somewhere.

Worried that is also might be a stuck Vampire Bat from the 'Where have all the birds gone?' mod. I'm going to try all these solutions later and see what I find.

I somehow fixed it by just running around that entire area and killing everything I found. Thanks guys.