hot water heat circulating pump

I have a hot water baseboard system running off my steam boiler, the circulating pump to the basebaord system is making a humming noise and I was told to replace the pump and that the cause of the pump making noise was air in the system. I hear the girgling in the baseboards when the heat comes up. the basebaord units do not have a air valve on them like the radiators. I want to put in a new circulating pump. How do I bleed the air out of system when I put the new pump inline. thanks to all