I would strongly suggest you find a new vet.
1. Failing to culture the infection is a big no no.
2. Prescribing oral baytril is another iffy move.
3. Telling you they could not do a culture because he wan't expressing any mucous or fluids.
This forum is rife with people spending a lot of money and time, not to mention subjecting their snakes to untold stress because of personable vets who couldn't handle a simple RI.
As Alex stated, doing a culture after the animal has been on several weeks of antibiotics most likely will not provide for an accurate test. If you can make the drive, there is an excellent reptile vet in Berkeley by the name of Ken Harkewicz. It may be worth your while to make an appointment with him.
Then again, were the noises continual or were they only heard in stress inducing situations?