I turned my (combi boiler powered) central heating on for the first time this season a few days ago. For a day or two it was fine, then the pump started making a loud grating noise. Bleeding the upstairs radiator cured the problem, but a few days later it came back. Now I find that turning on the upstairs raidator cures the problem (previously I just had the downstairs radiator turned on). I can turn the upstairs radiator off and on and the noise comes and goes instantly. Is having just one radiator turned on bad for pressurised systems? Would it help to increase/decrease the pressure?