noise when tap turned off

When I turn the cold tap off it goes `clunk` one loud knocking noise, its recent as never used to do this.
Any thoughts on how to cure?
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Airlock? Try turning on all the taps at once.
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wrote:

Just the one tap?
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On Thu, 12 Jan 2012 14:38:19 -0000, "Mr Pounder"

No, all the taps at once in the hope of letting the air out. That was what my plumber advised. Worth a try, I would suggest.
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Water hammer? Is the tap fed from the rising main?
Mike
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It will be the stopcock.

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Exactly, hence the question about the rising main!
Mike
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Okay, calm down and tell the OP how to fix it. New washer?

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