In a radiator system, steam travels up from a boiler and circulates through radiators, where it condenses into water and returns to the boiler. Gurgling sounds are caused by air trapped in the system or steam passing through accumulations of water.
Supply Valve Problem
Steam moves faster and takes up more space than water. The supply valve located at the top of your radiator makes sure the steam and water pass each other easily. A round knob opens and closes the valve. Water can corrode and loosen the seat of the valve. If that happens, you can turn the knob but the valve still won't work, and your radiator will gurgle. You might have to replace the valve.
Air Vent Problem
Radiators have a vent to let air out as the steam travels upward from the boiler. The air vent, usually found on the side of a radiator, often makes a hissing sound. This vent also lets air back in when the steam condenses, returning water to the boiler. If air can't get back in, the condensed water can't drain. Water collecting inside the system will make your radiator gurgle. The air vent contains a short piece of metal called the tongue that hangs inside the radiator and drains condensed water. If the tongue is bent or twisted, the vent won't drain and your radiator will gurgle. Replace the air vent.
Radiator Not Level
Over time, spitting steam can soften a wood floor, causing the radiator to list or lean slightly to one side. If the radiator doesn't drain properly, steam passing through accumulated water will make gurgling sounds. Radiator bottoms contain what is called a "mud leg" where sediment accumulates. If your radiator leans to one side, sediments can accumulate around the supply valve. You might think the valve is open when it is partly blocked by sediment. To fix the problem, remove your radiator and flush out the sediment.
The sections or loops of a radiator are connected by what are called push nipples. Acidic particles from the water can collect at the bottom of older, cast-iron radiators, weakening the push nipples and causing the radiators to sag. Sagging radiators collect water. Steam gurgles as it passes through the water. If that's your problem, replace the corroded push nipples or find a way to shim the sections to return the radiator to its natural shape.