Cloud serpents growl too as you fly around on them. A lot of the new mounts have different sound effects instead of/along with any wingbeat sounds. Companion pets do as well. Other than that, it might just be a bug. Blizz is preparing a new content patch for us. Something might have slipped in, or something else is not being paid as much attention to and needs tweaking.
Sound bugs are so common I don't hear them much anymore. Most recently, my gorilla tanking pet charged in and sounded like a raptor.
but also, just like a few min ago i started hearing even MORE growls, still not any pet sounds, completely new to my ears, but its louder and stronger.
like the first growl i heard "weak cat", now its more, "big angry cat"
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I had it frequently (as in every...half an hour or so?) while in the Jade Forest, on everyone. My druid, my hunter, mage, warlock, shaman, paladin, warrior, priest...but it wasn't always the same sound. It really seemed to be that the game just picked a recently heard sound and played it at random times. Usually it was a growl from a tiger (which I never killed!), but sometimes it became something else. A crane, a cloud serpent, sometimes even a saurok or hozen.
On a path, on a mount, sat in a town or in an inn, idling while waiting on LFD, whatever 'n wherever. It wasn't during combat or with anything happening close by, but sound did have to be enabled for it to ever occur. Don't think it happens anymore though, I'm not sure, 'cuz I don't really spend any time in that zone after the cloud serpent stuff was done.
To purr, or not to purr...