I don't have this boiler but on my combi the pressure ia around 1.5 bar when cold and higher when hot. Maybe the fitter meant it should be 1.5 when cold. Is there a red zone on the dial?
Rumbling could be several things - pump bearing?
You could phone the manufacturer and ask for tech support to check pressure settings are within specs. The pressure is initially set through a filling loop - a flexi pipe with a tap that fills the system (assuming it is a combi boiler).
Ant
November 2007