knocking pipes

Hi In central heating the boiler is absolutely fine. When we run any hot water tap you hear a whining sound from the boiler much like water running through or past a blockage. As you increase the flow on the tap it starts to cause a vibration in a pipe, I assume, which sounds like a drum...

I wonder if anyone could help me with this one please? A month or so ago, I fitted a new bath and new taps. They're fed by 22mm copper pipes. I have those braided flexible hoses for the last foot of pipe run to each tap. All seemed fine, but in this last week, there's been a loud hammering...