My DS is 7. He was doing it for some weeks - if not months, until recently. Especially when sat down, and especially if relaxed. Always at dinner time. When he was busy he was less likely to do it, but was still doing it, just with less of a frequency.
I was going to take him to the doctor, but my DH had a view it's not worth it, - he thought it's something like a very mild attack of asthma, and isn't worth being treated in any way. (DS had eczema and mild asthma as a baby, so there is likelihood of it surfacing again). As we're both working full-time, we decided not to bother, and it eventually disappeared on it's own.
I think it was triggered my stress / anxiety (although can't exactly say where anxiety itself came from), and when things settled, it went away.
For a 5 year old tension / anxiety may come from struggles with speech. A lot of kids stammer around this age, and this also fades away completely for many. It could be that there's too much going on in the little brain, and it developed this habit in an attempt to cope with it all.
Would be very interested to hear what doctors say, as we've not got round to visiting one.