Today when I was running around in the forests, I suddenly heard a zombie growling, I saw movement, but the movement turned out to be a rabbit, the sound came again, while I was moving towards the rabbit, and it got louder and seemed to originate from the rabbit itself, I looked around for quite some time afterwards to make sure that there wasn't a zombie there, but it seems the sound came from the rabbit, either that or there was a zombie spawned underground or something like that.

A couple of days ago I experienced something similar, when I heard a loud zombie-noise coming from some bushes, I looked around for quite some time but couldn't find any zombie. Now, I have no idea why the zombie-sound would accidentally come from a rabbit, so it might be more likely that there's a zombie spawned underground or something similar, but I thought I'd mention it anyway.

I have seen rabbits without getting the zombie-noises aswell though.