greenstar ri wall hung rsf gas-fired condensing bolier?

The boiler had a full service in april 2010 everything fine, until I put radiators on sunday last.Radiators seemed to come on fine and heat up then after a while got too hot turned down by thermostat in hall.Then on going upstairs could hear a loud clicking noise.Just tried it again fine until I turned down thermostat in hall then again clicking.Any help greatly appreciated.Turned off now as it worries me.