Confirmed.
They make sounds only when they in pacific condition. As soon as they get activated by you... thats all. No sounds. Apart from sounds you've been hit =).

Comme on guyes ( developers). Zeds are not perfect killers or hunters. They are f88ing stupid, they must yell when they made sure you are alive and eatable.

But never saw them invisible. ( its logical=) )

Completely agree, and as the additional fun - the astonishing is that if you try to circle it in old way and succeed somehow they disappear for split of the second when hitting you with the tentacles on their back in the same time...

Same here: I have noticed that zombies footsteps cannot be heard - when I am creeping slowly in a house, my footsteps sound like elephants, but zombies can charge in at full speed and make no noise. Similarly, a zombie chasing you in the forest makes no sound; I can hear the footsteps of other players near me, but not running zombies.

I seem to get these invisible zombies while mainly inland about 2km all the way up to the NW Airfield. They are silent until they hit you and cannot be killed (tried by spamming an entire AKM mag where the zombie was attacking me from)
They've been a bother since 0.42 but are far worse now in the new zombie update of 0.55, much faster, can attack from any direction and cannot be avoided by sneaking because you can walk right into them.

On top of the pre-existing teleporting zombie bug, this makes 0.55 close to unplayable for me, and I'm almost kitted out with the best weapons and gear in the game so far... kind of frustrating.

Yeah... Ezio Auditore da Firenze.