Some dogs simply snore. Could also be something like a reverse sneeze.
I wouldnt run off to the vet if the dog is otherwise fine. But I would get a video of it, and next time you're at the vet show them the video and ask them about it.
Snoring can be a concern, elongated soft pallet that could cause breathing issues, especially in dogs that are sick and potentially already having problems.
~Emergency Vet Tech
Berlin vom Spartanville 1/13/13
2008 - 7/23/12
"Take this trouble from me: Make sure my shepherd dog remains a working dog, for I have struggled all my life long for that aim." Max Von Stephanitz