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Bleeding the air out of the system may help reduce the noise, esp on the upper floors of a building.

I love the sound of old rads knocking. very nostalgic for me

this site has decent forums: http://www.heatinghelp.com/

i bought the book mentioned in there and it helped understand how steam heat works and what we can do to prevent knocking

one thing i have yet to do but it atop my list is: insulating the heating pipes that run across my basement ceiling

i just wish i could give you my forced air heat for your noisy radiators! i loved mine when i had them.

I don't even hear it anymore. I did look into bleeding the radiators once. It's usually not necessary, and kind of a bigger job than I would tackle with no professional, and I consider myself fairly handy.

Really K T G?

In my building... we just use a radiator key that you pick up for 70 cents at the hardware store - insert it in the tiny hole/valve it is designed for... and turn to the left to let the air out until water follows. Once the water passes through - turn the valve tight closed with the key and viola!

All you need is an old towel to catch less than an ounce of water per radiator and it's easy as pie.

There may be a difference. Our building is only 60 years old - and I'm told it's hot water heat rather than steam heat although I can't imagine what the difference is between the former and the latter.

clickchick - I am mostly unfamiliar with how it works, but when I first moved in, I was concerned that there was a serious problem. In a previous apartment, there was a serious leak in the apartment above mine which leaked, and bulged my ceiling and a big slab of plaster fell on my bed. So it is with the (1) bad experience with a possible different cause and (2) ignorance whether this banging means anything I need to address or not, that I went looking for answers.

The how-to of it that I read involved plumbers and wrenches and very strong people to make sure that the end was lifted to prevent air, and gallons of boiling water - the kind of DIY where you end up making a huge mess and end up paying a pro to resolve anyway. Did not read anywhere about this cheap key you mention, but was mostly assured a certain amount of banging and hissing is normal and nothing to worry about. At least now I know about this key and try that too. I don't even notice the banging, but still, it would probably be best to minimize it. The vertical pipes still bang, there's really nothing I can do about it.

I love that sound! To me it's the sound of cozy and snuggle time - hot chocolate and a good book :)

Uh... You guys are talking about two different systems here; you only bleed a hot water or hydronic system. Steam systems are not bled, as they are empty except when the steam is entering and condensing back as water. The pressure relief valve may need to be replaced (they can be cleaned in boiling water, but usually it's cheaper to replace it). The banging is caused by the radiator's not being level and the steam reacting with undrained condensate, however some knocking is normal, since the heat will expand the pipes a bit, and the hiss is the air being forced out through the valve. The advice about the valve is absolutely correct - steam heat is on OR off, no in between - the valve will leak causing damage otherwise. There are also fancier steam systems with two pipes, which I believe is called a vacuum system which pulls the steam through - this is an almost silent system.

Another tip; if you are in a position to change out steam radiators, DO not replace them (unless they are leaky or just way too awkward) with the modern radiators which are now available - you need the cast iron to hold latent heat and radiate it back out after the steam cycle is complete. Some larger systems run more continuously and can handle a lighter gage radiator. My parents were just in the position of replacing their more "modern" convector units with more original cast iron to provide better heating.