My adult male cat has suddenly started having involuntary hissing fits. What could it be & how do I fix it?

It started suddenly a few weeks ago. He had come in to lay down for the night and started hissing short fast hisses. It was almost like a hairball & hiccups all at once. That first time I thought he was gonna throw up so I sent him back outside. Since then I have watched & it happens at all hours of the day & night and has no obvious trigger. He seems so miserable when he is doing it & often sneezes when he's done (which sometimes produces some phlegm). I don't know if it is something like a sinus infection or what. Perhaps he is trying to "clear his throat"? I am lost. It not only bothers him, but it makes getting a good night's sleep hard for us too. LOL! If anyone has had this problem I would love a bit of advice. Thanks to all!