While steam radiators are no longer a very popular way of heating houses, partly because the radiators themselves are extremely heavy, and lighter, easier to install systems have become much more widespread, many older homes still use them. If you use a steam-heat system in your home, you have probably grown accustomed to high-pitched whistles and knocking sounds emanating from the radiators. These are not the sounds of a normally functioning radiator, which should be silent. The good news is that quieting noisy radiators is quick and painless.
If you hear high-pitched whistling when the radiator is running on full steam, you need a new air vent. An air vent releases air so steam can escape the radiator, but that whistling sound tells you that instead of steam, air is escaping. The air vent is the little metal piece that sticks out the side of the radiator. To replace it, you just need to untwist the old one and twist on a new one, after wrapping the threads in Teflon tape.
If steam is escaping from the radiator and causing additional whistling, you have to repack the main valve. First, close the valve then remove the screw that holds the valve knob in place. While pushing down on the screwdriver, simultaneously pull up on the knob. Remove the knob and unscrew the gasket. Repack the threads with graphite packing by wrapping a small piece around the gasket stem. Replace the gasket and the knob. This will create a tight seal and keep in the steam.
During normal use of the radiator, keep the valve knob fully open or fully closed, never in between. Keeping the valve partially closed or partially open does not mean that you are letting more or less heat into the room; instead, it means that you are allowing steam and water to mix, which causes bubbles. The bubbles pop in the metal pipes or in the radiator and cause that banging, clanging sound.
Switch out the vent and replace it with one that has a smaller hold if the room is too warm. Check other rooms to be sure the valves are all open---if the other radiators are shut off, all the steam will be sent to the one open radiator, producing more heat than necessary.
Switch out the vent and replace it with one that has a larger hold if the room is too cold. In addition, it's a good idea to insulate the pipes running from the boiler to the radiators so the system doesn't lose heat on its way to any particular radiator.