Sometimes I hear a thrumming vibrating sound, but I haven't been able to determine what it is. It's not the ringing sound made by Nirnroot. It sounds more like the thrumming of machinery or magic. Two locations (they aren't the only ones, but two I remember) where I heard it were: on the road near Fort Greymoor and in the Kolskeggr Mine. The sound seemed to be restricted to a particular vicinity. If I moved away, the sound faded and died. If I moved back in the direction of the sound, it got louder, but still no sign of what was causing it. What could it be?

Maybe the earthquakes in the Reach when you would get close to a particular location?

hmmm... well it doesn't sound like an earthquake...

What does an earthquake sound like?

I don't know what an earthquake (in Skyrim) sounds like, but an earthquake happens and then it is over, whereas the thrumming sound I hear is constant and doesn't go away unless I move away from the area I hear it in.