When you open any chest in 16w05b, you have a big chance to hear the opening sound lower-pitched and normal at the same time.
That lower-pitched sound shouldn't be there of course.
This is only for 16w05b, not 16w05a.
The same thing happens when you close the chest.

Just do /playsound minecraft:block.chest.open master @p and you'll hear it happening again.