Yep David I think it originates at the pump, pump inlet or outlet. That black mould you describe can fill the pump causing the pump to fail in pushing water through, even though its spinning the impeller is just caveatting as water isn't getting to it or possibly out of it due to a tough blockage. Sometimes the impeller can be so stuck that the shaft spins free in the impeller blades Usually denoted by a higher spin sound than you would normally hear.
