I thought mine was coughing/sneezing - but it turned out to be gape worm..... After the sneeze/cough, does the hen keep its mouth open a bit, no sound, (the 'gape')? If so its gapeworm and needs Flubinvet (vet diagnosis by postmortem! Wasnt quite what I had in mind when I took mine to the vet....)...
If not (no gape), I'm sure someone else will be along with their experiences soon. Good luck, Fi