Hi, Lance. First, check to see that the inside stop valve that controls the sillcock is completely open. A "sillcock" is the "official" name of your outside faucet. So that you can shut off the water to the sillcock in the winter so the water in the pipe does not freeze, there will be a valve inside the house to allow you to shut off the water to the sillcock. Be sure that this valve is opened completely. If you are in a part of the country where freezing pipes are possible, you want to turn off this valve in the fall and open the sillcock. Reverse the process in the spring. Not having this valve completely open may cause the vibration.
If that does not work, change the washer in the sillcock. This is just like changing a faucet washer. Close the inside valve. Remove the sillcock bonnet and the stem and bonnet should come off. An assortment of flat faucet washers from a home center should give you a new rubber washer of the correct size. Replace the rubber washer and brass screw. Because of the many different designs of sillcocks, I can't give you more than general instructions but this should be pretty easy if you have done any plumbing at all. Be aware to that there are freezeproof sillcocks that have very long stems that reach way back into the house so the actual valve is past where the water would freeze in the winter. The odds on you having that style aren't great but I should let you know this just in case. You would still replace the washer in the same basic way.
If this still doesn't work, the next step would probably be replacing the washer in the inside stop valve. Same general instructions as for the sillcock. You will probably have to shut off all the water to the house.
One of these will probably fix it but if you have any concern about doing any of this, call a pro. This should be pretty quick and easy work and should not cost much more than just the fee for having him come out in the first place.