I know the vibrations are coming from the direction of the hot water heater, and if I can remember correctly, it started right after the hot water heater was replaced. But I'm not for sure, this is in my parents house and my father is the type of guy who thinks he can fix any thing. He has tried draining and recharging the plumbing system as in the case of water hammer with no results except perhaps a little more knocking, he won't call a plumber till something bursts. But maybe if I can give him an idea of what might be wrong he might.
The sound is like a jack hammer and vibrates the pipes and faucet but its only when turning it on, when you turn the water up higher is stops and after a few min. you can turn it down with out the knocking.