When turning on hot water faucet in bathtub at a certain point when turning handle it will vibrate and make a loud blow horn noise. Why is it doing this?

Sounds like you have an older tub faucet with a rubber washer which is now vobrating as water flows passed it. Shut off the main, open the faucet stem and replace the washer. Resurfacing the seats when doing the washer is a good idea too. HowStuffWorks "How To Repair a Faucet Valve Seat"