I think she has a cough. Once in a while is normal - just like people. Because this is happening every night, I think the problem is worthy of having your vet check her out. How old is she and what breed?
My parent's dog is a German Shepherd/ Lab mix. She is 15 years old this year. Ever since she was about 6 or so she made snorting noises sometimes when we would take her outside. The vet told us that she probably had an allergy. When we kept her in with the air conditioning on, she's fine. About last year she started making snorting noises at night when she was sleeping as well. Her allergy has gotten worse and she developed a tumor in her nasal cavity. We keep her on a regimen of benedryl tablets. maybe your vet might find an allergy as well.