It’s 3 in the morning, and you’re asleep on a futon in your old, tiny apartment, when suddenly there’s a BANG!

What sounds like a hammer clanging against metal pulls you out of a dream about a giant talking turtle. And surely it’s not ghosts, you say to yourself.

And you’re right. It’s not ghosts. It’s probably your radiator.

But how does a hollow hunk of metal make such a loud noise? It all has to do with tiny amounts of water flying around in there. Steam radiators have been used to heat buildings since the 19th century.

And it’s a relatively simple setup: you have a central boiler that boils water. The steam the boiler makes takes up way more space than it did as a liquid. As it expands, it pushes itself through a series of pipes.

Now assuming you’ve opened the intake valve to let that steam into the radiator, it will rush in and push the air out through a small air vent on the opposite end. When there’s enough steam inside the radiator, the pressure causes the vent to close, trapping the steam inside. That air vent can be one source of annoying radiator noises.

If you’re hearing a whistling sound, the vent might be gunked up with limescale, a residue from minerals in the water. Banging, on the other hand, comes from what’s happening inside the radiator as the steam cools down and the metal heats up. Eventually, some of the molecules in the steam lose enough heat to condense back into a liquid.

Usually, gravity pulls that liquid back into the pipe it came from, and it drips down to the boiler to eventually be heated up again. But those tiny little bits of condensed water can make a banging sound loud enough to interrupt your conversation with that turtle. If some of the water can’t leave the radiator — say, because the metal is sagging, or your old floor has given way and now the radiator’s tilted in the wrong direction — incoming steam will whip it up and throw it at incredibly high speeds into the radiator’s inner walls.

This is even more likely to happen if the intake valve is only partially open, becausethere’s less room for the steam to enter and the water to escape at the same time. And hammer-like sounds can also happen when that stuck liquid water cools some of the steam enough to make it condense very quickly. Suddenly it’s taking up much less space, and there’s a vacuum where the steam just was.

Some of the water nearby gets pulled in to fill that void, which ends up ramming it into the nearest bit of metal. That’s why the banging noises tend to happen more when you’ve just turned the heat on. After the system’s been running for a while, a lot of the condensed water will heat up and become steam itself, leaving less to hurl around.

But condensed water isn’t the only possible source of banging. Since your metal radiator and the pipes attached to it are trying to expand as they heat up and contract as they cool down, they might be loudly bumping into something nearby. It should be pretty easy to tell if that’s happening just by looking.

But no matter what’s causing it, if your radiator’s making loud noises, you should probably call a plumber not the Ghostbusters. Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming. If you want to submit questions to be answered, go to patreon.com/scishow .