... and you can't remember what it is or where you heard it???
Have you ever changed the instrumentation for that tune (again, all in your head...) so you're even more confused than before about the source???
They say it is the journey, not the destination that matters.... Well, I'm having a heck of a journey!
That's how I discovered the Italian Opera. I'm still not certain, but I think that when I was 3 or 4 years old, I must have heard snippets of Puccini and Verdi during TV ads for cleaning detergent or canned spaghetti products, and growing up, I had this wonderful, intensely melodic music dancing through my head, and had no idea where it came from.
When watching the movie Moonstruck, I thought "WOW! I have got to check this stuff out! And that exploration opened up the whole opera thing for me.