If you have old-fashioned radiators in your house that make gurgling noises, this is an indication that your radiator is not pitched correctly. Radiators needs to be pitched so that the steam in the radiator goes out the steam pipe or into the steam trap (depending on your type of system). If the pitch is off, the steam gets caught in the radiator and it goes through the water... which is what leads to the gurgling.

So if you hear gurgling noises in your radiator, use shims under the legs to adjust the pitch of your radiator to get the gurgling to stop.