unvented hot water central heating problem boiler potterson ?

boiler firing ok, watter silender ok but heat not comming to rads watterpump ok ( strong ) pipes hot around the pump , pump making funny gushing noise like its sucking the system had prev been draind down in prep for sale of house we filled it up and this is were we are please if any could help , iv put the 2 way port valve in the manual open position and this other white box took off and manualy put in the open position , watter is coming out pump wen centre screw is removed header tank is full , mains water on, rads full no air , element in cylnder ok ,