Coughing Coughing? Hacking?

Hi everyone, well Rocky has been making very weird sounds today and I am starting to get worried. This morning I heard this cough or choking sound from up stairs and I ran down to check on my piggies but both were fine (looking at me like "Hey wheres my food!) I thought I might have just heard something else. I came back home from work and I heard that cough/ choking sound again and saw Rocky shake his head a little (like if he was rubbing it on the floor?) But...as far as I know his eating, pooping, acting normal and "looks" healthy. Any one know what might be causing this cough or choking sound? He doesn't have anything in his mouth and I don't think he was eating when he made that sound. Thanks