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'It doesn't sound like your dog's breathing issues are related to his liver mass but labradors often get a condition called laryngeal paralysis. This condition may be associated with age and degeneration of peripheral nerves. http://www.petmd.com/dog/conditions/respiratory/c_dg_laryngeal_disease
Classically we hear noisy inspiration but in any older dog (labs especially) with a history of difficult, loud or abnormal breathing, a laryngeal exam should be performed to rule this out. additionally, there may be other laryngeal issues that cause noisy breathing including a mass or obstruction, laryngeal collapse or tracheal dysfunction.
It is good that he is not yet in distress, as often laryngeal issues are not discovered until the dog is brought to the emergency room for difficulty breathing making for a life threatening episode. I recommend discussing a work up with your veterinarian including a sedated laryngeal exam, radiographs and possibly referral to a specialist for fluoroscopy or laryngoscopy/tracheoscopy or other indicated diagnostics to look for a cause of the noise you hear before he gets to the point that he gets into distress.
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