When we turn the hot water on, alone, in the bath tub we hear a very loud vibrating sounds in the pipes. This only happens when we turn the hot water on to the spigot, not when we run the shower, or if the cold water is also turned on. It only happens in the tub, not in the sink. If we keep turning the faucet on higher, it goes away. We've checked to ensure there is no place in the pipes that they are banging up against anything. What could be wrong???