Same thing here, thought it was a possible char database reset. Occurred around 2pm EST last night.
May 10 2016
I had this issue, then I realized I was accidently alt-tabbing to the desktop. Changing the inventory key to a different key resolved it for me.
Would slide through walls and floors.
It almost feels to me like the zombies are under ground or something. It seems like they follow you.
I can confirm this as well, I figured it was a simple placeholder sound for some other content coming in the future.
Observed same behavior with wood axe.