It sounds like there may be "more to the story" to me. Did you go ballistic back? Competition Obedience instructors can be pretty harsh sometimes...I think it comes with the territory. BUT, as a student you need to suck it up sometimes and understand where they are coming from, too. In this situation, it looks like you are in the market for a new instructor, as it sounds like the bridge has burned.
We have a couple of those duck trolling retrievers in our competition obedience class, and they are VOCAL, LOL, but it's a "bark" vocal, not a "growl" vocal...AND....the owner works consistently with teaching her dog NOT to vocalize, which is a big part of why she's in the class to begin with. My dog has learned to completely disregard the trollers, but I think she'd have a much harder time ignoring a growl (it's a growl, whether you call it a grumble or not) coming from the dog next to her in the long down.
So....harmless or not, you'd be wise to work on a silent down.
