I mostly play on vanilla official servers.
Sometimes i hear wolves barking (not howling) and when i close distance to the bark sound it's hind (most of the times 2) making the noise.
I have evidence once they ran away the wolves bark seems to follow them or disappear if they went out of range.
I found it funny but maybe it's confusing for beginners since they would maybe run away from the sound but it is actually no threat.