M.P.
This is completely normal. My son did the same thing around this age. Our doctor told us at the time to not call attention to it. Try to ignore. If you make a big deal out of it, YOU will cause him to attach a label or problem to it. My son outgrew it in a few months or so. It came suddenly and then it dissappeared suddenly. I have been told it is developmental around that age. I think it is because their vocabulary is incresing significantly and sometimes they get "stuck" trying to get the right words out while they're learning and processing other new words.
Hope this helps!
Updated
P.S. Our doctor also said do not tell him to "take a breath" or "breath" or "stop" or whatever you feel that you want to tell him to do. Just let him work it out and do NOT call attention to it. HE does NOT think there is anything wrong....