I love Istanbul and have been there a few times, but one thing I forgot to work out was the tune that seems to be playing everywhere.
I know it sounds vague, but basically it's a tune they play in speakers in some of the tourist shops in Taksim, or also you'll find some of the street kids with accordions (or some instrument) playing it too.
It's an instrumental with a sort of sad/sombre tone and it must be something famous as I'd heard it a few times. The tune sticks in your head.
Would be great if someone had a name or link to it.