Spraying any of the NPC characters. They'll yell something like "NOOOOOO!!!" and a string of random angry gibberish. Except for the Nokis and the Sunflowers and Peach; the Nokis just retreat into their shells, and the sunflowers mumble gibberish that sounds like "Oh, I'm loving it!" Peach just blocks the spray with her umbrella, and the biggest sunflower giggles an adorable sound like "lo-wanny-hee-hee-hah!"
Talk to Toadsworth. Sometimes he'll just scream what can only be described as "YABLABBABLABBA!". He also makes this sound when you spray him (after he spins around and almost loses his balance).
The boss fight music tracks are either this or the Most Annoying Sound. They make use of rather whimsical instruments that either intentionally make a farce out of the battle or simply grate on your eardrums.
There's two bossthemes, aside from the remix of the second used for Phantamanta and the theme for the final boss fight against Bowser.