Since we had the builders in and they drained the system to fix a new
radiator there is a tapping sound that begins when the boiler is
firing. It starts fast and slows down over about 10 secs and is causing
SO to get quite cross when she is attempting to get to sleep.

Originaly the pressure in the system was just over 1 and the builders
left it at just over 2. I bled one radiator until the pressure was back
down to 1 to see if the noise went away but it didn't.

I think the noise is coming from the pipe area where they exit the
boiler but can't be too sure. I'm pretty sure that before the work all
radiator valves were fully open.

What is the correct pressure for the system? Does anyone know how I can
get rid of the tapping?

Thanks for any advice.
Rich.