knocking hot water pipe

I recently bought a house and have a problem with the hot water in my upstairs guest bathroom. When I turn on the hot water, it will flow normally for a second or two, then the flow will drastically reduce and a knocking sound occurs. When I turn off the water, the noise stops. I can then turn the hot water back on and the same think will occurr again. I was told it could be air in the pipes, but I'm not sure this is the case. Either way, I'm not sure how to remedy the problem. My main concern is that there is a leak and that there is no hot water for my guests in the sink. I also thought it was strange that it was only this sink and only the hot water. Any suggestions?
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snipped-for-privacy@mail.utexas.edu says...

The knocking could be a water hammer effect. Do a google search. These usually happen when flow stops abruptly. The flow-stoppage could be occuring due to a bad washer. I would replace the washer in that hot water valve and see if that fixes it.
Some older houses had "air trap" pipe extensions that were supposed to hold a buble of air in a capped off pipe segment, and the air is relatively compressible so would damp out any sudden flow stoppages. These days it's considered better to put in water-hammer suppression fittings. Again, google will help.
Marc
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Christit wrote:

It is more likely to be caused by no air in the pipes than air in the pipes :-)
The knock could be water hammer and you may need to turn off the water at the main then open all the water valves (make sure the hots are open) and let them drain. You may take this time to drain a couple of gallons of water from your hot water heater as well (do this last and turn off the heater's thermostate before turning off the water)
Now when that does not do it (it may and it is cheap and easy to do so it is a good place to start) I am guessing it is an older fixture. If so I suspect a loose washer.
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Check the valves on that hot water line.. If you get a noise when the water is running the chances are high that you have some FOD (Foreign Object Debris) in your line. It could be in the shut off valve under the sink or other.
Water hammer will occur after the water is shut off.
Randy
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