Check the stops at the fixture and others. Open the faucet and close the stops till you find the one that has the vibrating washer. If and when you find it replace the washer. It is usually a 00 (double aught) washer. Or you can replace the stop. Probably the easier option if you have never replaced a stop washer. They are pressed on. Strong fingers required and a good o-ring pick helps. Could also be some other device but this is the most common cause.