The Dukes (#991) put an electric car horn on
Uncomfortable Hunk of Metal '03. When we started climbing the ramp in autonomous, the horn gave a "beep beep" and then our driver had control of it which made for some entertaining battles for bins and/or King of the Hill spots.
Nobody dinged us for it during inspection at three different events so I'm pretty sure it's okay. Keep in mind that overdoing it would make it very annoying, though, and that it only takes one person's complaint to make it illegal for all of us.
Good luck to you's!