I think you need to be more specific about how the song goes. There is a lot of songs that go dun dun dun.
the crazy frog song
Hail to the chief is played upon the President's appearance at formal functions.
The name of the song you are looking for is "Songbird" by Kenny G. It was rated the #1 smooth jazz song of all time by AOL
the dun dun song (so stupid)
June bayze
Flight of the Valkeries
Moonlight Sonata
Für Elise - Beethoven
I don't really know what your looking for but I think it might be Beethoven's 5th Symphony. It goes: Dun dun dun dun... Dun dun dun! Dun dun dun.. Dun dun dun dun!. Or it may be a piano song? If it's the piano song then I don't really know its name, Sorry. I am looking for it to so that's why I ended up here! :)
Kashmir, by Led Zeppelin.
Maybe Crazy Train by Ozzy Osbourne?
I believe you are thinking of crazy train by Ozzy Osbourne.