I would assume that there is a strainer on the suction side of the pump. I have experienced a similiar problem on #6 oil pumps. We replaced the pump and motor as you stated you have and the noise continued. We kept the same suction strainer on the new pump and that was the problem. I went around...
I would check the boiler for carry-over, check steam traps for proper operation. At around 400psi silica voltalizes at an increasing rate. The problem with softeners (sodium zeolite) is that they don't get rid of silica, only mixed bed demin systems and reverse osmosis due so properly.
When burning hydrogen rich fuel, for every 1 pound of hydrogen you burn it makes 9 pounds of water due to chemical process. A good way to determine the amount of air you need is to take the higher heating value of the fuel and divide by 1300: HHV/1300. It gived you pound of air per pound of fuel.
