You say this is something he’s done in the past, so I guess your girlfriend has discussed it with her vet at one of his routine checkups? What did the vet say about it?
The dog is getting routine checkups by a vet, right?
If not, please find a good vet and bring him in for a checkup immediately, and thereafter on a yearly basis at least. If you/your girlfriend can’t afford routine checkups, start looking into re-homing the dog now.
This does require professional, expert attention because it could be serious. Maybe it isn’t, but don’t you want to know for sure?
Considering this is not a new phenomenon, if the dog is in all other ways fine – and you know this because you observe him closely every day and do regular home checkups – you can wait until his next routine vet appointment to ask about it. In the mean time, take a movie of the dog making this noise, and bring it with you when you go to the vet’s. (If you don’t have a movie recorder, borrow one from a friend, post the movie online, and ask your vet to look at it next time you see him.) If your dog starts to exhibit any other bad signs, bring him in ASAP.
On the other hand, if you don’t know how to do a home checkup, and/or if you don’t know what counts as a bad sign, bring the dog in immediately.
Please don’t let this end at fluther. Strange people on the internet can talk out of their asses or, even worse, purposefully give incorrect advice for malicious purposes. Sure, strange people on the internet can also give really excellent advice – can be experts themselves. The question is, how do you know which is which? The internet is great for getting a variety of opinions and experience, but not as an end-all be-all, not for important issues such as this.