I've been making funny noises on brass instruments since I was a kid. It wasn't exactly spontaneous- lots of pros used to make the occasional funny sound, like horse whinneys and so on. The guy in Spike Jones' band may have been my original inspiration.
I did it in high school a lot because a) it would keep us amused while the woodwinds were going over and over and over and over stuff, and b) because we'd come up with new stuff and try to copy it.
I recently discovered that I could play "backwards", which has been a mite hilarious.
Anybody else got any goodies they'd like to share?