Hey there Ed, this is a really common problem. The 2 things to conquer first:
1. Understand how the lips and air column work when playing and that you don’t have to engage the larynx when playing – I’m pretty sure you have this sorted.
2. Be aware of the feeling or sensation of the open throat. Have you seen the “body’s concert hall breath” video? If not, it’s a MUST watch!
Once your understanding of the system and desired feeling are in check, then it is a matter of looking for the trigger point. A what point in the process of playing does it happen?
Go through the tissue, visualiser (or fingers), mouthpiece, leadpipe then instrument steps, very slowly with your eyes closed. You will feel where the grunt kicks in.
If you totally believe in the process and surrender to it, meaning the whole sympathetic oscillation process, then it is simply a matter of experimenting and resisting the urge when it wants to happen.
Keep returning to the breath with the finger to reinforce the sensation. Let me know how you go.