They never do this when you bring them to the vet. The dog repeatedly sucks are in through his nose, making gross noises, like people sometimes do before they spit. Some dogs do it off and on for months or years without developing any apparent problems. It can be a sign of an infection or foreign body in the nose, but those usually cause a discharge, and usually get worse as time goes by. Dogs who are too thin.
Some dogs are naturally thin, especially until middle age.
As long as your dog is active, runs and plays, has a good hair coat, has normal bowel movements and doesn't drink an excessive amount of water, and is not losing weight, don't worry about it. It may be a good idea to weigh him once a week or so for a while. Dogs who don't eat enough. The guidelines on dog food packages are written by people whose profit depends on how much you feed your dog. They also produce commercials to make you think every dog should come running to the food dish and bury his head in it, slurping, crunching and swallowing happily, until the dish is empty.