One of my babies does that, but I call it "huffing" instead of hissing because they do it with their mouth closed.

My little boy, Nago, always huffs when I pick him up. It's just his way of telling the big, bad monster [me], that he could do some serious damage if he had to.

He's my little sweetie. He's never struck at me, but his huffing scares hubby to death, even though he's not even big enough to get your finger in his mouth.

Gale