If Slim is eating and drinking normally, and normally active, it does not sound like a severe problem.
However, it is hard to say what exactly is the problem. Does he make the noise when he inhales, or exhales? Dogs can do something called "reverse sneezing
" which sounds dreadful, and is not nearly as bad as it sounds. The noise with reverse sneezing is made when the dog is inhaling.
If the noise occurs when Slim exhales, he is either coughing, or trying to expel something from the nose
He could have allergies, dryness in the nasal passages, or an infection.
A couple of things you could try. One would be to use nasal saline drops, like NaSal, and you put one to two drops into each nostril, once to twice daily, to moisturize the nasal sinuses. Another is a Chinese herb called Pi Min Kan Wan, which helps with irritation of the nasal passages, and works well in cases of allergies and reverse sneezing.
Pi Min Kan Wan is available at many herbal and "health food stores' and pharmacies. The dose is one to two pills twice daily for 2 weeks, depending whether Slim is under 35 pounds, or over that weight.
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Good luck with Slim. Rebecca