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knocking sound?

hi
my boiler make's a knocking sound when the central heating starts up i have checked all the radiators and none seem to have any air in them.
can you help me please

my boiler is an ideal classic lxff & ff
thankyou
kim, February 2010
check pump not working so water not circulate and boiler is overheating if sound comes after a while than scale in the boiler

mr K, February 2010
Sounds like air in the system...Try bleeding your rads.

CHUM, February 2010