Hot water system I assume? Might be an issue with the feed valve not opening. Might need a new air vent. (See pic)

Are all the registers getting hot? Because if it's gotten airbound you're going to need a bucket and a garden hose.

If it's a steam radiator gurgling I'm going to look at the pressuretrol and probably turn it down and look at the pitch of the piping.

Is the air vent near the boiler? There isn't one in their bathroom. It has a separate thermostat, and the gurgling seems like it's only in that register.