Where a piece of music is bugging you so relentlessly that you believe you can actually hear it. There's a radio in the far side of my office, barely audible from where I'm sitting, but I swear that annoying Madonna song with the Blondie sample has been on a 24 hour loop for the last fortnight.
Occasionally this happens when I'm very tired, starting to drift off to sleep, I think about some music and I start to actually hear it, but find I can control it with my mind. It's quite cool! This is way more annoying though. I think most likely it's a noise coming from a machine or something which from a distance vaguely resembles the Abba sample in question (which is quite a distinctive yet bland sound, not dissimilar to a dot matrix printer), and my overactive mind is deranging it in the most awful ways. IF only the song was DRMd this would never have happened, as the copy protection is designed to prevent this sort of thing.
My greatest fear is that this has now become a mental habit that will continue to taunt me for the remainder of my days. A living hell.