A "popping" noise is a sign of a RI. Also a clicking sound is a sign of a RI. A snake should not have snot or anything kind of fluid coming out of it's mouth or nose. I'd call the vet back tomorrow and tell them what's going on.

Not something you want to let go since RI's will not just go away. If left untreated it will end up killing the snake, if that is what the snake has.

Up the humidity and heat in it's enclosure.

EDIT:
I'm confused. Is this Sula you are talking about or one of your BP's?