You know they can't hear you right? It kinda doesn't matter if you whisper sweet nothings or if you curse a blue streak, you can still startle the hell out of a sleeping snake with your movements - and since they don't have eyelids either, being able to tell when they are sleeping takes a little finesse, and observation on your part of some very subtle cues.
And while I say they can't hear you, they can definitely detect sounds in lower frequencies or louder intensities because of the minute vibrations that we usually ignore.
Either way, you experienced a hiss. You probably just rudely awakened him, and he hissed because he was startled and felt threatened.
I would advise you some caution though, about how much you handle your snake around feeding time - for the snake's sake. If you handle your snake after he eats, you may cause him to vomit - and snake vomit is oh-so-NOT-pleasant I promise! As I've explained to my husband, it's the smell you've never smelled that you want to ensure you never smell! Since snakes have a fairly slow metabolism (compared to mammals), as a bare minimum, I leave mine the heck alone for 2 days after they eat. Then, because I still worry about "tummy upset" and about being pooped on, I like to wait about 4 days after feeding before I really get them out for handling. Every snake is different though, and I have one that poops like clockwork 5 days after eating. Then aside from all of this pleasantness, if you handle your snake immediately before offerring food or even between prey items, you can expect to get bitten, or to put your snake off his feed.
For this reason I am a big advocate of feeding snakes in enclosures separate from the ones they live in - this way they don't associate hands reaching into their cage with food, just with you handling them. They learn to associate food with the other container, and will expect food once they are put in the other container!
You asked about prey size, and quoted what I refer to as "the girth rule" which I heavily disagree with (eventhough the majority of keepers seem to live by it). Ball Pythons' proportions are not something that makes me comfortable using "the girth rule," since their necks are SO tiny and skinny, and their midsections tend to be quite a bit larger. The reason I don't like this method is that I once had a ratsnake that managed to tear the soft tissue at the corners of her mouth (when we did size by "the girth rule" and obviously misjudged something). Every feeding after the initial injury, she would tear her mouth again, and it was terrible looking, couldn't feel great, and it was not fun to treat. For that snake, we had to resort to feeding prey items that were smaller that the width of her head, to avoid stressing her scars. What I advocate for determining prey size in Ball Pythons and other species is to look at the width of the jaw at it's widest point, and to give prey items that are about 1.5 times that width. Yes, ultimately, they CAN handle larger prey than that, but this isn't some kind of ironman competition, and I think it's better to err on the side of caution, for the health of the snake.
Just thought I'd give you my 2 cents' worth. Welcome to the addiction. I grew up keeping snakes and other reptiles (Dad had turtles in the house before I was born), and after spending a whopping total of four years of my life without snakes, finally realized that this is an incurable addiction.
Good luck to you!
~Rebecca
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0.1 Dumeril's Boa '04 (Courtney)
1.2 Ball Pythons
[1.0 '05 Orange Hypo (Specter)]
[0.1 '05 Het Orange Hypo (Sylvia)]
[0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)