Sound coming from baseboard pipe (in 1 room only) when hydronic heat is on.

There is a constant sound coming from 1 room on the second floor from a pipe that is inside the baseboard radiator. For some reason all the other room's don't have this sound. The sound is from the pipe and not the baseboard. It only comes on when the heat is on and the pump is pumping water through the system. 2 weeks ago the expansion tank was changed and the system purged. The heat has not been on since that time. Last year this was did not exist. It's a constant dull sound (like if something was moving through the pipe, maybe like light humming but not really). There is no vibration. The pressure in the system is 14 Cold. What could this be?