Noises can be fairly common in plumbing supply lines. If a washer in a faucet or valve is loose, you'll hear it rattling or knocking. If the sound occurs when you open and close faucets rapidly, it generally means pipes are loose, and can be corrected by anchoring pipes more securely. If it really bothers you, you can add air chambers at the end of long pipe runs. Their installation will probably require a plumbing professional.