If it is not a digital servo, disconnect the push rod from the servo horn, or just pull off the servo horn, and see if the rod is binding anywhere; see if there is any friction, or if you have to slightly bend the rod to reconnect it to the servo. The rod needs to move fairly freely.

You can try connecting the servo in another slot in the receiver to make sure it's not something in the radio or receiver.

As mentioned, digital servos make noise all the time.