gurgling noise

Help Mr. Wizard!
I've got a two pipe steam system in an older building. The steam and condensate mains run around the interior perimeter of the building in a concrete trench. In one area, inside a stair closet, you can hear water gurgling in a 3/4 inch steam riser. The riser goes up from the main about 5 feet to a radiator in the stairway. When you set the stat for that radiator to max heat, the gurgling goes away. I've replaced the trap, but that didn't help. Any suggestions? Thanks for you advice!