Steam Heat Hissing Sound

My house has a one-pipe steam heating system. Every time the heat comes up, I hear a hissing noise comming from the steam vent. My question is if it is normal for there to be a constant hissing noise while the heat is comming up or should it be mostly silent? If there should'nt be a hissing noise, I have to replace the steam vents right? Thanks.
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Single Pipe Steam Heat at: http://www.bbc.co.uk/dna/h2g2/A533819
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When water turns to steam, it expands, which blows air out of the air vents as steam makes its way to the radiators. The air vents should close when the steam gets to them and the steam condensing to water draws more steam to the radiator. As the boiler cycles, more air might be pushed to the radiators. But if the air vents are blowing steam, they are not operating properly, or the pressure is too high. When the boiler shuts off, the remaining steam shrinks to water and draws air back in.
Two secrets for having a quiet and efficient system are main air vents on the pipes in the basement (to quickly and quietly fill the mains), and having the system operate at a low enough pressure (not more than 1.5 to 2 psi max.). If the air vents are very noisy or whistle, either your main vents are stuck closed (or you have none), or the pressure is too high.
For more info, see http://www.heatinghelp.com/ and The Wall forum there. They have excellent books about how steam works.
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