I know her whistle is normal because I trust the breeder who produced her implicitly, and she said the girl has made that sound since she was a hatchling. I have also had her for quite a few months now with no change/increase in the sound.

A culture swab isn't taken of the whole mouth. The cotton swab is rubbed directly on the glottis, preferably when the snake takes a breath. Yes, it is possible to get some mouth flora in there, but it is more focused than just swabbing the mouth.