Oh radiators, those overworked, underappreciated winter necessities. Who ever really notices them...? Unless they start to gurgle and groan, and make all sorts of grotesque noises.
As an integral part of your home's efficient functioning, they periodically need attention and care, too.
If you can hear water moving through your radiators, there is air trapped inside your pipes, reducing the efficiency of heat delivery to your home. It is time to "bleed" your radiators to let the air escape, leaving a closed circuit of hot water only.
This one is so simple, this is going to be a very short post.
If you have floorboard radiators, like we do, gently remove the cover and locate the bolt (While your cover is off, go the extra measure and dust off the plates).
Make sure your heat is on and actually running, otherwise this is a wasted exercise. The hot water needs to be moving through the pipes for it to push the trapped air out. (If you have the taller, cast iron radiators, the process is the same, you just don't have to remove a cover.)
Set a glass below the bolt so that the water that drips out will not drip onto the floor.
Loosen the bolt until you hear a hiss, and then carefully loosen the bolt a little more, but not so much the bolt falls out of the threads. Keep the bolt loosened as long as there is a periodic hissing while the heat is on and running.
Once the hissing stops, wipe the bolt with a rag and retighten the bolt. Depending on how much air is trapped in there, it can take up to an hour with the heat constantly running, but it should take around 15-20 minutes normally.
Replace the cover. DONE. All you should hear when the heat kicks on now is the gentle popping or tapping as your pipes and radiator plates are heating up.
Congratulations, now your radiators will be functioning much more efficiently. And they will promptly begin fading from your notice, until you forget again that they are there, keeping your house at a steady, livable temperature.