I recently fitted a new heat only worcester cdi boiler. On the flow coming out the boiler the 1st tee is the vent, 2nd cold feed,3rd to hw cylinder then the flow carry's on to the rads. When the boiler is fired up fo HW only it heats up very quickly then it sound like something is rattling in the pipe at the moment i have the by-pass opened a little more than usual and this stops the noise and the customer is happy, but i still want to sort it. There is an A.A.V at the cylinder.