An outdoor faucet that makes a loud vibrating noise when it's turned on or off probably has a worn out faucet washer. You can easily replace it without removing the entire faucet. First, turn off the water to the faucet. Then use a wrench to remove the retaining nut that's attached to the sill cock (Photo 1).
Slide the handle and stem assembly out of the sill cock. Remove the screw at the end of the stem (Photo 2) and remove the washer. Buy a new washer that matches the old one at any hardware store. Then reassemble the faucet. Occasionally the washer is fine but the screw holding it is loose. If so, put a drop of Threadlocker, or a similar product, (sold at hardware stores) on the threads and tighten it. Some faucets have a spring-loaded sleeve near the washer. If you have this type, replace the entire faucet.