A small plumbing problem.... novice plumber to the rescue(?)

Recently the GF had significany rennovations done to her home. It involved a new bathroom. The fixtures remained basically in the same locations, but the plumbing had to be adjusted very slightly. New taps, nozzles, etc. There were no signifant changes made that I amaware of.
The issue is when she turns off the COLD water quickly there is a thump-thump behind the shower wall, but it does not repeat itself. It is difficult to tell, but it seems to come from above the faucet assembly. The water feeds from below.
The same happens if the sink in this bath and the adjoining bath or the toilet flow stops suddenly - it seems to be the cold water in all cases.
I do not think it happens with hot water, nor does it happen if the water is turned off more gradually.
Is there a possibility it could be the shower valve assembly, or a loose pipe? I am hoping there is some way to try to repair it without going through walls.
Any imput/observations/approaches to try/or further diagnostic methods to narrow it down more would be greatly appreciated!!!!
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That's classic water hammer. You can find lots of DIY web sites now that you know the right name.
I would start by draining the pipes to recharge any existing hammer chambers: Turn off the water at the meter, open all the faucets and let the water drain for 30 minutes, close all the faucets, turn the water back on at the meter. When you open each faucet thereafter, air will come out for a few seconds.
On the other hand, tell the remodeler you have this problem. He should fix it under warranty.
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Steve Bell
New Life Home Improvement
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This good advice omits only the explanation. Water hammer is shock in the water pipes (transmitted everywhere in the system because water is incompressible.) The building code now requires at least one shock absorber in every system, a vertical pipe, capped at the top, and full of air. Shock in the pipes can push water up into this pipe (because the air compresses). But in time the vertical pipe can become full of water (waterlogged) thus has no compressible air at the top to act as the shock absorber. The solution is to drain the whole pipe system (not the tanks that supply it) so that the vertical pipe empties and can resume its function.
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Don Phillipson
Carlsbad Springs
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Modern shock absorbers use a bladder system to avoid the absorption of the air into water. They've shown them a time or two on Ask This Old House.
Puckdropper
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Thank you very much everyone. I will try that. I will shut the main H2O off and totally drain by letting it all sit for a bit with all faucets, etc open to see if that will help.....
IIRC, I did see a copper 'stand-off' at the top of each pipe in the shower which I assume would be the air 'absorber' column you are speaking of. (of course this was before all the ceramic tile was put up....).
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On Wed, 11 Feb 2009 12:11:16 -0800 (PST), " snipped-for-privacy@hotmail.com"

    That what they are.
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*If the renovations were done less than a year ago, call the plumber back to see what his remedy is.
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