Sorry I didn't respond to your post earlier but my laptop is toast and I am limited on my access to the internet.  Anyway, my Lil Guy would do that.  Sometimes it actually scared me because he would have a sneezing fit that seemed to last FOREVER and I was afraid he couldn't get a breath.  It never seemed to bother him, he would go about his normal activities and never acted "sick".  My Peanut did that occasionally too and would also at times sound like she was trying to get phlegm up (sorry I know kinda gross).

Towards the end of Lil Guy's life he had a constant discharge from his nose (and I mean constant--I would have to make him sleep on blankets and I was constantly changing/washing them).  He would be better after being on antibiotics but it would always come back in about a week.  I always wondered if he really had something like the feline herpes virus which can be dormant in a lot of cats and when he became geriatric he couldn't fight it off anymore.

Hoping it turns out to be nothing and Jack is just reacting to some allergens.

Marla and tri-dogs Daisy & Biscuit (and Maybelle cat, too!)

I would definitely get him checked out if it's out of the blue.  Could be weather or something in the air.  Do you have anything new that has a smell to it?  Perfume, laundry detergent, etc?  I'm just guessing here.  

My cat Loki has sneezing fits all the time, but he's always had them and eye/nose discharge.  His sinuses have been messed up since he was a kitten and got infections.  His name was Sneezy before I adopted him...poor kid.  When Loki had problems that actually needed medicine he always had really irritated eyes and was visibly uncomfortable.  But still if Jack is acting different than usual, something must be up even if it's minor.

Feel better Jack!!