Loud water sounds when tub faucet is turned on
The noise may be caused by harmonic vibrations. Try adjusting the flow by opening the tap less, the downside is that it will take longer for the tub to fill.

Other options tend to be more invasive, like opening up the walls and/or ceiling and strapping the supply piping tightly.

You could try a new tub spout, one which delivers more or less water per time unit, or has different flow characteristics.