The Bug:
While aggressive toward mobs or players, tamed wolves used to growl. However, at a time I can't pinpoint off the top of my head, they stopped doing so. I'd like to say around 1.6 or 1.7, but I'm not 100% sure.
I looked at release notes and on the Minecraft wiki and saw nothing specifically mentioning "-removed tamed wolves growling" or something of the nature. As such I can only assume it's a bug; wolf growling has always been very glitchy in the past.
Steps to reproduce:
1. Tame a wolf
2. Have the dog attack something it can fight
3. It will not growl, just bark. Contrast this with wild wolves attacking something, which do growl.
Screenshots added; subtitles show the noises being made
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Looking at this again, it would seem that although tamed wolves do now growl while attacking mobs, their tails also "straighten" like aggressive wild wolves which I do not recall this being the case in older versions of Minecraft.
This seems to have been fixed at some point despite it never being mentioned on a changelog. Tamed wolves growl at their targets now.