Trapped air in hot water baseboard heating pipes is a common problem. The trapped air can make a loud noise in your pipes while the water is circulating through them. To remedy this problem, purge the air out of the pipes. Removing the air will involve removing some of the water from the system. This water can be quite dirty, especially if it has been circulating in the system for a long period.
Turn off power to the boiler. Locate the correct circuit breaker in the service panel and turn it off. Make note of the water pressure in your heating pipes.
Open all of your zone valves that run to each of your heating zones. Close all of the shutoff valves to these zones.
Screw a length of garden hose to a drain spigot on the return line that runs to your boiler. Direct the garden hose to a floor drain or bucket.
Open the auto-makeup valve and the spigot to which the hose is connected at the same time. Allow the water to drain out of the hose until no more air bubbles are seen. Keep an eye on the water pressure and do not allow it to rise above 25 psi. If it does, release the auto-makeup valve momentarily until the pressure drops.
Release the auto-makeup valve and close the spigot. Repeat Steps 3 and 4 for each zone in your system.
Unscrew the garden hose and put it away. Close each zone valve and open all of the shutoff valves. Verify that the water pressure is the same in the system as it was before you bled out the lines.
Turn the power back on to the boiler.