Might not be an RI- snakes can get a bit of water in their nostrils when they drink, or a tiny bit of skin gets left behind (in the outer nostril) from their most recent shed, & that can make a faint clicking sound when the snake breathes. When he's in shed, you should raise the humidity to 60-70%- that will help. If it's too dry, it's more likely that bits of skin get left behind- & that might be what you're hearing now from a previous shed. I'd hold off on the vet visit unless you see bubbles in his mouth, extra saliva, or hear more noises than the minor & occasional clicks you're hearing now- like more constant whistling & crackling. While it's always best to catch an RI "early", you also don't want to go to the vet for nothing- it's stressful for the snake.