Respiratory infection help

[slowhite03]

So I made a post awhile back about how my variable kingsnake showed signs of a ri, well I took him to the vet and got shots for 3 weeks and took out the aspen and started the shots. Now he went into shed during this time and before he shed he was making a lot of popping noise, my initial reason for thinking it was a ri. After the shed, he was fine. He was no longer making the popping noise and seemed good to go. I took him back to the petsmart vet, yeah I know not the best vet but only one in town. Vet gave him a clean bill of health. Took the poor guy home and reapplied aspen to his tank and thought he was grand. Well a week after that, he started making the popping noise again, needless to say I wasn't happy. I took him to an actual vet and he gave him a thorough check thru. He checked his mouth, heart, and lungs. Said nothing was wrong that he could see. The vet recommended that he may have grown an allergy to the aspen. I took him home and put him on paper towels and it seemed to work but today I picked him up to change his paper towels and he started "popping" again. It wasn't often but more periodically. I know none of you are vets and are reading this really long post with a ton of grammar mistakes but has anyone expierenced anything like this before?