Does anyone hear a radio playing?

I don't think anyone's suggesting that it's a symptom. I think what it is is that it's just something many of us experience more frequently than the average bear (and maybe we're just more "tuned in" on ups, on account of taking in more sensory stuff). I'd liken it to rage that way. It certainly is not a diagnostic criteria, or symptom, but does seem to be something we experience at a greater rate than the general population.

As to your original question, psychedhedone, I've never heard an explanation of why it happens exactly. When my psych mentioned the fan thing, I thought about it and figured that maybe it was just our brains trying to put nonsensical sounds into a more familiar form. (Like, machine noises are, really, quite new to our brains compared to speech or music(!)) Reading later, I stumbled across a word for that phenomenon, but cannot think of it at all (and search terms to try to look it up were too broad apparently). I remember it because it was a tiny bit disappointing to learn that it wasn't an original thought, lol! Very silly. (Maybe I was hypo and feeling grandiose at the time. )

But in the absence of such a device, no idea!