My buddy said I need a new butterfly for my intake. Ford focus 03, makes a rattling noise when engine is revved. What is a 'butterfly'?

Where can I get one

Added (1). Also, could this be a cause of a vacuum leak

The butterfly is the actual throttle valve in the throttle body. The butterfly valve itself is very unlikely to be bad, more likely is that the throttle body is worn at the throttle shaft bores causing a vacuum leak. You can get rebuilt throttle bodies at the local auto parts store. Be sure to get instructions on how to properly install and calibrate it.

What L.N. Said.

Do you know for certain the throttle body is making the noise?

Hi so your buddy is another idiot. When a car gets to be as this one is now 14 years old it's a lot worse than just a
Butterfly that makes the noise. He whole engine is well past it's scrappage date.

It's the round plate that moves when you press the gas pedal to allow more air into the motor. Some motors have adjustable intake runners that have a butterfly in them too so I don't know on your motor if it has one of them that may also be the source of the rattle You don't want to go looking for something you have no idea about or how to install. If you're half way smart that is!