I've had both. Hearing things/music with fans and the like and without any machinery. When I asked one of my former psychs about it, she said it is very common and not anything to worry about. In fact, when I described what I was hearing, her first question was, "was there a fan on?"

I think it is part of the phenomenon that also can happen visually. Our brains want to make sense of things, to make them recognizable. It tries to make sense even of randomness.

Something similar to this happened to me just tonight. Saw a shape at a bit of a distance. My brain first interpreted it as a person. Getting closer, turned out to be a shrub. Lol.