Kitten sounded like a train

I have a kitten that's about 4 or 5 months old that we're trying to introduce to our much older cat (it isn't going super well) Occasionally I'm able to hold our older cat in my arms while sitting on our bed, and she doesn't seem to care if the kitten tries to get to her. That was going on tonight, but then she (older cat) decided she would be brave and step off on her own. Of course he jumped up to try and attack her, and she hissed and swatted at him, the usual. But then he (the kitten) did something weird. He got down like he was getting ready to pounce, but started blasting little breaths out of his nose, increasing in speed as he did. When he did this she hissed and swatted at him and he stopped. Then he did the exact same thing again, with the same result.

Not sure if I'm describing this well enough, he genuinely sounded like a train that was starting up and I've never heard him do this. Has anyone heard of this before? Is this normal behavior? Had a hard time looking it up due to not knowing how to phrase it