Some dogs, notably rottweilers, make similar vocalizations even when relaxed. Search for 'rottie rumble’ to find clips with examples.

I don't know anything about wolves really, but with dogs, most individual components of body language are pretty ambiguous. For example,

  - many dogs will rapidly wag their tails just before getting into a vicious fight (it doesn't just mean 'happy')
  - dogs will generally bare teeth as a warning/threat, but some will also show their teeth as a greeting (Google 'heeler smile')
  - some stress/calming signals (e.g., pulling ears back or lowered tail) can occur as normal parts of safe, mutual play

To get a sense of what a dog is feeling and how it might act, you really have to integrate all of its body language into a single context.

All of that is to say that if someone who works at a respectable wolf habitat/rescue tells me that a growl-y vocalization from a wolf is not always aggressive, I don't have a hard time believing that.

(I'd love to learn more about wolf body language and how it differs from that of domestic dogs!)