Costanza learns to whistle. It turns out he never knew how to whistle, but he took whistling classes, and he won't stop whistling for no reason. Not whistling a song, no, just spontaneous little "Woo" whistles, like when ordering food and pointing at the item, or when a pretty woman walks by, and Jerry will feel very uncomfortable and annoyed and all.
Jerry: "Whaaat? There are whistling classes? Who doesn't know how to whistle?"
George: "Many people don't. Jimmy Carter can't. Ronald Reagan couldn't until he learned how to in a whistling class, look it up."
Jerry: "Get out of here."
George: "Woo."
Later in the show, Elaine walks in, and instead of greeting her George whistles another short "Woo".
Elaine: "Uh?"
Jerry: "George took whistling classes and now he won't stop whistling".
Elaine: "Whistling classes? Who doesn't know how to whistle?"
Jerry: "I know!"
Elaine: "I know you do."
George: "Woo woo"
Kramer walks in looking all excited
Kramer: "Woo!"
Jerry: "What??"
Kramer: "Woo!" "Woo woo!"
George: "Oh you took the classes, great uh?"
Kramer: "Woooooo!"