Sounds like air in the system and may happen because system has been off for a while.Fire up the system and go to each radiator in turn with a bleed key and old towel. Undo the bleed screw a little and water or water and air will hiss out.I did say hiss,honest. Do this to each rad in turn and see the outcome. It may need doing a few times to completly bleed all the air out. Caution you don't get scalded as the water should be well hot.
Jim M
August 2008
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