i replaced the prv as it was letting by, recharged the pressure to 1.5, and turned back on, after 20-30 seconds i could hear "gurgling" then a dull knocking, sounds like air in system, bled all rads(all fine)took cap off pumphead and that was turning ok. noise does stop when i turn on hot tap but returns a few seconds after turning off. its an old boiler and does not have(that i can see) the auto air vent,though it does have a small tube sticking out the top next to the reset button with a "washing up liquid bottle top" cap. any suggestions are v welcome