My rats hiccup all the time, whenever they're happy or excited about something usually. When they were younger you could barely hear it, but after awhile, one of the boys probably started realizing that noise=attention, and now he's VERY loud about it when he does hiccup (he 'chirps'). You can hear him all the way on the other side of the apartment. I think my other rat may have learned it from him, as he didn't do it for a long time, and when he did it was inaudible. After awhile he started doing it louder, but he's still pretty quiet.
These are the first rats I've had that have have really made much of any noise beside little squeaks while they're playing/grooming.