Name this song?
 

I've had the first few seconds of a song going through my head for the past few days, but I can't place it. I think it was from a game, but it may have been from anime. In any case it sounds like an ending credits song.
Here's my lousy rendition of what I remember, this was the melody of the vocal part because I don't remember anything behind it. Lyrics were Japanese, with a female vocalist. I'm pretty sure the first word is "itsuka", but the next few syllables just sound like "da-da do-o" in my head.
I think it is something well known so someone should be able to recognize it despite my shitty keyboard skills.