circulator rattling

I have a gas fired Steam Boiler. This boiler provides steam to radiators in the main part of my home. It also supplies hot water to base board heating in an extention to the house. The hot water suction is below the water line of the boiler, pumps through a circulator, up one floor to approx 60 feet of baseboard and back to the boiler through the steam return line. (steam is a one pipe system) The circulator makes a rattling noise that I suspect comes from the hot water flashing in the pump. This happens only when the water temp is 190 degrees or above, but below 212. The water temp usually peaks at 205. Is there anything I can do to prevent this from happening.