My hydronic heating opperates with two Grundfos circulation pumps one circulates water to one end of the house and the smaller one circulates water to the other end of the house, the smaller pump became noisey and the radiators soon became cold. I have bleed the system and checked that the pump was operating (electricly and the impaller was intact and spinning on the shaft) Could the pump be at fault ? What else can go wrong with these pumps?