Cat mouth breathing, clicking sound?

• Signalment: Cat, American Shorthair, 8 years old, male, neutered, 11lbs.

• Clinical signs: Mouth breathing, clicking sounds from mouth when breathing or sniffing

• Length of time you have seen these changes: around 4 months

• Your general location: Wisconsin

Hello!

My 8 year old boy has been exhibiting these weird breathing symptoms for months now. I've taken him to two different vets and both say his lungs sound fine and that there's nothing to worry about. They said to give him 5mg of Zyrtec a day, which I've been doing for over a month and have seen no noticible results with. This video was taken a few hours after his daily Zyrtec dose.

I can't shake the feeling that something is wrong, because there's no reason for him to be breathing like this even with daily Zyrtec, but the vets won't take me seriously.

Any ideas on what it might be so I can bring it up to the vet? Thank you!