The whistling may be nothing or it may be that his nasal passages are too dry or it could be the beginnings of an upper resperitory infection. It's a little hard to tell just from the description. The first thing you want to do is make sure your humidity levels are good, Most care sheets will tell you that a humidity level of 50-60% is good and I have found this to be correct. Also, make sure your temps are right. I have had good results with winter temps of 85 hot end 82 cool end during the day and 82 hot end 78 cool end at night and summer temps of 87-88 hot end, 83-84 cool end in the day and 84-85 hot end, 81-82 cool end at night. If it persists or gets worse you may need do see a vet but first make sure the environment is correct.
As for switching him to ft rats, it may be easier to switch him to ft mice first. Then, once he is used to taking ft mice you can use a mouse or dirty mouse bedding to scent the ft rat. The switch from live to ft can be really easy with some snakes and really difficult with others. Same goes for switching from mice to rats. It really depends on the snake.
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"yeah, eagles have feathers hanging off of them too and I don't smoke --- out of them!" PS it was a dream catcher!

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