This morning when the heating came on, we got a progressively louder knocking noise from the water pipes near the water pump. I switched off the heating and it stopped. Now, with more time to look I have switched on the heating again, for 20-30 minutes there was no unusual noise, then the knocking started again and lasted for 5 or so minutes before I could get into the cubbyhole where the pump is. By the time I'd got in, the knocking had stopped, there was a swooshing noise but having not spent much time with my head near the water pump, this could be normal. Is the pump knacks red? Is there an air bubble in the pipes?

You should not hear the sound of water running through the pipes. What is the temperature & pressure on the boiler?