Yep, just like Dustette said, sometimes they can get hiccups. Mine used to get them a lot more when they were younger, especially after waking up and/or eating. Tooth grinding is a sign that they are relaxed and happy, though if he does it constantly, you might want to check to see if he seems ill. Sometimes they grind their teeth a lot more when they aren't feeling well.
Bruxxing is also how they keep their teeth ground down... AS long as its relaxed you are good to go, if its grinding and the rat seems stressed, it could be stress, pain or illness. If you see the eyes pop in and out from the socket a bit its called boggling and can sometimes happen at the end of a brux movement.
As for hiccups, are they silent and do they happen often? They should be silent and if not, or they happen a lot keep an eye out for a URI pending.