Well, there's a lot of things it could be and it's impossible to diagnose certain respiratory issues long distance and without testing because they have the same symptoms but different causes.
Are the sinuses or the area around the eye, or between the eyes and nostrils, swollen at all or flaring with breathing?
Any bubbling around the mouth or nose at any time, even right after it's been drinking?
Does it breathe with its mouth open habitually?
Not sure what you mean by nasal sound, that could be a lot of things from clicking sounds due to obstruction to the sound of forced breathing, actual vocalizations, or the sounds of mucous bubbling in the airways. Can you describe it a bit better?
Best wishes.