I was going to suggest it may be a hiss. You probably wouldn't hear anything since really no sound comes out but I have seen quite a few people alarmed by it when they had never seen it before.
I dont think its hot in there at all. Boo doesnt spend that much time with them in the basket either unless she is nursing or bathing them. He seems to pant on and off just like a puppy. I also noticed he has a puppy belly where the others dont.
Heres another Q, (I know I am a worry wart lol), do kittens jerk and tremor like they are dreaming when they are sleeping and fall over a lot when they stand up?
Yes, small kittens under 6 months old will jerk sometimes when sleeping. And yes up till about 8 weeks old they are not too steady on their feet. They tumble and trip a lot....but its cute to see them attempt to race and not be coordinated for a few weeks.
If any of these "signs" persist after 2-3 months old, THEN you have neurological problems.
I was told that panting is an indicator of a rapid heartbeat or anxiety. After talking to a vet about when Tobee in the car while we were driving and he started to pant like a dog.
As others have suggested, it sounds like a very normal kitten hiss. Since kittens are born both blind and deaf, they rely heavily on sense of smell and touch. When they smell something different from their mother/siblings, they attempt to hiss at it. Hissing serves two purposes; 1) to assist in identifying the unfamiliar smell by using the scent glands in their mouths, and 2) to be intimidating to whatever is making that smell. (As if newborn kittens could BE intimidating!! *smile*)
i have seen kirra pant before when it was hot and if its a pant and not a hiss, their tongue would hang out some (like a dog) its funny looking on a cat, cause well you dont expect the same look lol.
if in doubt if its a pant or hiss, and your sure temps are good where the cat is and shouldnt be hot, call your vet, best advice we can give, when in doubt call the vet they can "see" better than we.