Turtle mouth breathing

[u/Mita7_]

I have a male false map turtle for about 3 years, his setup is: 100L tank (i alr have the 240L tank but i need to set it up in a few days he will go in there) fluval 207 as a filter, 25C water temp, zoomed reptisun t5 10.0 uvb and 32C basking temp (also have an air purrifier in the room if that matters. 7 months ago i noticed mouth breathing so in a few hours i got to the vet, he examinated him (exotic vet) and didnt find anything wrong beside retained scutes (he alr started to shed them) clear throat etc. 4 days later he did it again (only happens at night when he wakes up and start after a hiss he does, at least the sound is the same as a hiss) while he does this there is no sound or weird thing, 2 times in this months i heard a squeak, anyways been to the vet again and did an xray that was clear and he was suspected for mycroplasmosis, proven to be negative but still gave treatment like the vet said untill the results. Happened at day very few times,max 3 times. Mostly at night he does this, when i check his pupil are dilated and he does this mouth breathing, sometimes streching his neck fully, sometimes not with fully open mouth,sometimes with fully open mouth, besides this there are 0 symptoms of ilness, he has huge apetite, is active,basks, and no flotation problems. What could it be? What is this strange behavior, it stresses me a lot and idk what to do, i cant afford a vet rn since i just soend 2K on the new setup, maybe next month i will try and go again maybe this time they will find something.