Skeletons make feint noteblock sounds when spawned, idle, or walking around. It's in addition to the rattling bone sounds, and happens during creative, survival, and survival with cheats. I've found this not only during gameplay with optifine and a resource pack, but also with optifine alone, resource pack alone, and neither.

I even went to the website, downloaded a new copy MC, threw it in an empty folder on desktop, and moved my .minecraft out of appdata. I have no mods, no maps, no optifine, nothing more than 100% vanilla minecraft like this. My findings? Noteblock skeletons. Everywhere.

I've always noticed this for as long as I remember, but never thought about reporting it as I didn't know it was a bug.

Uploading audio without video is a bit of a pain for me since I can't seem to do one without the other.. and splitting them always gives me errors and/or corrupts the files, so if someone else can attest to this, it'd be really appreciated.

Proof has been added in way of Youtube video. Do take note.
https://www.youtube.com/watch?v=dOW9GBIUXTY