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Hello. Thank you for asking your question. My name is***** will try to help.
Anytime it sounds like there is phlegm in the lungs it would be a good idea to have your cat seen. Especially if you hear any wheezing or the gums look pale or purplish.
He may need a chest x-ray to rule out asthma, heart disease, bronchitis, or pneumonia. Because all of those can present with the same symptoms it is important to have a vet listen to his lungs and possibly take a chest x-ray to see what is going on inside.
I would try to get him to the vet today or tomorrow.
I would not just monitor him and see if he heals.
As you probably know heart disease, asthma, and pneumonia could be life threatening if left untreated.
Does that help? Do you have any additional questions?