Thanks for the advice and info everyone!

I will videotape Duncan next time he does this, if ever. He didn't do it last night at all.
And I will make a vet appt for him too.
Duncan and I are so close, I am so aware of his every behavior and attitude and feeling...the grass eating is new as is the "reverse sneezing" so it definitely warrants a trip to the vet for me! Any unusual behavior by him is watched very carefully and I usually drive the veterinarians nuts by bringing him in all the time!!!
I will let you all know how it goes!

Judy J., first of all welcome to Pet Talk!! Second, I'm sorry to hear that your little pup has these reverse sneezes, they are upsetting to hear. Everything I read about this reverse sneezing says it is much more common in small dogs. That is another worry since Duncan is a very LARGE dog.