I have Keston boiler with pressurized CH system. After I cleaned the CH with Fernox Restorer and flushed it then added Fernox Protector, my pump (Grundfos) seemed to get a bit noisier than before (lauder humming noise). I've bled the system several times and it doesn't seem that there is air in it. I've set the pressure to 1.4-1.6 when hot. Any ideas what may cause the pump to be noisier and how to fix it?