XP Orbs make 'walking' sounds when moving.

Steps to Reproduce:

-Grab some Bottle o' Enchanting and throw them (or kill some mobs).
-Stand near the XP Orbs (close enough for them to move towards you).
-(Recommended/optional): Turn off music.

If you listen very closely, you will hear 'walking' sounds coming from the XP Orbs as they move towards you. This is more noticable if there's a large cluster of them.