If your vet prescribe anti-biotics just in case of X Y Z here is my advice change vet.
Just like for human or other animals antibiotics should not be prescribe in case but after a diagnostic is made, the vet should have started with a culture not a prescription.
There are reasons why snakes can make that sound, stuck shed, but also the snake itself, I had a female years ago (Black Pastel) that was making a whistling sound and she did so since I hatched her, some people who had held her though she had a RI because it did resemble the sound of a RI.
So find a new vet one that is specialized in reptiles and make an appointment to confirm or eliminate any doubt if you think there is something wrong,