IMO, the two you've already mentioned would make the most sense.
One time I worked IT at a hospital and had a doctor who complained his computer was "breathing". Upon going to his office to see it in person and clarify what he meant, sure enough it was making "breathing" noises from the speakers. He had multiple windows open in the web browser and one of them was on a Star Wars page. The sounds were of Darth Vader's breathing ...
Sometime people complain about ridiculous problems where they are the only ones to blame.

I wouldn't be surprised if it was spilt water inside the lip of the frame.