My boa is about a year old and is audibly breathing. He has done this twice now, once when I was holding him outside of his enclosure a couple of days ago and again today while I was feeding him. I am not a professional snake keeper so I do not know the difference between hissing and breathing sounds. He did not seem annoyed when I was holding him the other day and wasn’t acting any different. When I was feeding him he missed his food and grabbed a leaf in his enclosure and while he was attached to it is when I noticed his breathing. He is getting into blue shed stage if that helps at all.
Thanks 
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There was a thread not that long ago with a similar situation, down to the pending shed. Someone speculated a piece of shed was causing the breathing noise. After the snake soon after shed, the wheezing noise went away. As long as the snake isn’t breathing with an open mouth and there aren’t bubbles coming out when it breathes and there aren’t other symptoms, I would say wait it out and see if it goes away after it sheds.
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There was thread on this same topic a little while ago and I think the reason for the noticeable breathing is the shedding process over the face of your snake. It is nothing to worry about and it should go away with the shed……. 
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Sometimes I could hear them breathe audibly cause i touched them🥲i guess ur snake is fine dont worry.Not so usually breathing loud is okay:)
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