That is actually pretty cool. Are you a musician/composer in any form? If not...I think you could be without too much effort. I would love to have the ability to sing harmony on the spot...I know the theory well enough to write harmonized parts, but not in real-time because it's not intuitive to me. And when I have a song in my head, it's usually just the main vocal line my attention can hold. With a LOT of effort I can "hear" chords or two parts in counterpoint, but I have to work hard at it.

All this, too, is anecdotal evidence, and evidence of perhaps nothing more than my own strangeness.

I can imagine hearing imagined sounds like you do, maybe because it's something I wish I could do...although you find it annoying, so maybe I should revise my expectations. I do know that up until about age 11, when I was completely tone-deaf, I had almost no ability to hold a tune in my head..."songs" stuck in my head consisted of the lyrics, in rhythm, but in a sort of monotone. Which is how I would then sing them, which is why everyone said I was tone deaf.