Could it be reverse sneezing? Poppy does this and so did my last dog. It's as if they can't quite catch their breath, their middle area really contracts with the effort of it and it's a strange grunty, breathy sound, really hard to describe. If you google it there are some videos comparing it to dog coughing.
An easy test of it is to hold your finger over their nose to force them to breath through their mouth, if that works then it's probably reverse sneezing. It's something to do with their soft palate/oesophagus and by breathing through their nose it forces everything back to normal. Poppy did this quite a lot recently but after a couple of times covering her nose it has stopped. I think she recognises the feeling now and immediately breathes through her mouth to fix it (cos last time it happened as soon as I put my finger to her nose she seemed to remember to breath through her mouth so I guess she does it automatically now).
If this doesn't fix it or if you're worried then get to the vet.