Noisy breathing in domestic cats indicates an obstructed airway. The airway that I’m referring to is the passage through the nose (or mouth), the back of the mouth, down the trachea (windpipe) and into the bronchail tubes of the lungs. If there is an obstruction in this airway breathing might be noisy and the classic reason for an obstruction is an upper respiratory disease either viral (cat cold or flu – herpes virus perhaps) or bacterial (bacterial pneumonia). Another reason might be a physical obstruction.
Distorted nasal passages
I have to mention right away the purebred Persian cat. The modern version of this cat breed with their characteristic flat face is likely to breathe noisily because their flat face distorts the airways from the nose to the back of the mouth. Flat-faced cats and dogs are known to have breathing difficulties. It’s one of those contentious points which feeds back to whether breeders of companion animals should be allowed to create inherently unhealthy animals such as the flat-faced bulldog and the flat-faced Persian cat. So that’s the first reason, if you are living with a modern, highly selectively bred, fancy looking Persian cat they might have noisy breathing for the reason stated. Note: owners of flat-faced animals don’t mind about the known health problems when adopting but they might mind later on.
I’ve mentioned upper respiratory infections which can cause noisy breathing in any animal because the infection obstructs the airways. Croupy breathing refers to obstructed breathing caused by an infection of the upper airway otherwise known as a “croup”. The sound is a characteristic barking cough. The larynx i.e. voicebox and the windpipe and bronchial tubes might be swollen due to the infection.
A cat owner might confuse noisy breathing with panting which is also more noisy than normal. It is, however, a normal cooling process and is used a lot more in dogs. A cat might pant after exercise or when stressed, indeed frightened, when perhaps travelling in a car to their veterinarian. Or a cat might have overheated and could be suffering from heat stroke, a very serious condition.
Wheezing is a form of noisy breathing. It’s a whistling sound which occurs on the in and out breaths. A veterinarian would say that it indicates a narrowing in the bronchial tubes which are below the trachea and in the lungs. They are referred to as airways or bronchi.
I think this is rare but it will cause a range of symtoms including difficulty in breathing, wheezing, coughing and hacking, rapid breathing, coughing or hacking, and open-mouthed breathing. Link to feline asthma.
P.S. This is probably an incomplete list. Please consult a veterinarian.