A radiator makes a gurgling sound
If the radiator gurgles, make sure the supply valve is fully opened.
Check the radiator with a six-inch level and make sure it pitches back toward the supply valve. If necessary, shim the radiator.
Correct the sag by shimming the radiator in the center. If the push nipples have corroded to the point where they leak, you'll have to replace them (if you can find replacements).
I've heard of people who have successfully repaired leaks in cast-iron steam radiators with epoxy. Success depends on where the leak is and its severity. If you can't repair the leak, you'll have to replace the radiator.
Remove the radiator and flush it out.
Check the air vent. Is it installed sideways or upside down? Some people turn the air vent upside down when they don't want any more heat. Check, too, to see if the air vent is installed on an extension pipe. Condensate may build up inside the air vent and keep it from operating as it should.
Remove the air vent from the radiator and check its tongue. The tongue is the short piece of metal that extends from the vent and hangs inside the radiator. Its job is to drain condensate from the vent. If its twisted or broken, the vent won't drain and the radiator may gurgle.