Some retics whistle naturally as they breathe. I would pay close attention to it though.

Never heard of treating mites with a shot. If that is a viable treatment these days, the mites might be killed on the snake itself but various mite life stages and their eggs will remain in the cage. If you have other cages and reptiles there is a very good chance they will become infested with mites. I highly recommend getting a can of Prevent-A-Mite and following the directions on the can exactly. Most important is not to spray the snake directly but treat the substrate and give it time to dry. This product will kill all mite life stages including eggs. It does take some time for it to work. Treating all cages is a good idea. I see you are currently using newspaper for the retic and that or indented kraft paper is what I recommend. Loose wood or bark substrate often harbor mites and should be treated before used. Bill
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...three years ago it was just another snake cult...
The Retic is King.
Anacondas-the other Dark Side.
Afrocks-the dark side of the Dark Side.