LRI is as people stated (using the wrong term URI). Excess saliva, bubbling saliva at the back of the throat, gaping, popping sound when breathing.
Since your vet said his lungs were clear he doesn't have it. If he keeps gapping then you need to get a lung wash to retrieve a sample and...
I would seriously doubt it was too much humidity unless its cold air mist while its winter.
I had a similar problem and cured it with WARM humidity. Throw him in the shower occasionally (ie in the bathroom on the shower rail or a vine draped into the shower but not allow water to hit him)...
Sometimes they clear their eyes by rolling it. Could be something in there. While many say its normal it bothers me as their eyes are fragile.
What I do and its been confirmed by a vet is to get eye wash and gently hold the eye open and flush it. I've done this many times over the years. The...
Sooo cool. I used to feed Karma grapes and he loved them but his system couldn't take it later in life and when he had trouble passing food grapes always did the trick. ;)
Great news Jann.
A diet is good. As Luie gets older he'll live longer by being leaner. I should've put Karma on a diet (350g) but he looked so happy being big. I've seen very healthy Panthers at 7 years because the keeper kept it on the lean side.