Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
It sounds like you are describing a condition we call "idiopathic head tremors." We see it occasionally in bully breed dogs- bulldogs
, pits, rotties, boxers, dobies. They will often be at rest and the head starts to tremor. It can last a few seconds or many minutes.
There is no known cause for these tremors. They also don't cause any problems in these dogs. For the most part, we just ignore them. They tend to cause more stress on the owners than they do the dog.
Here's are a couple videos of these, you can see if this is what your dog is doing:
I hope this helps, let me know if you have any other questions.