Think this one might nail it Chicken... Water pipe knock
Subject: Water Pipe Knock
I get a knock in my hot water pipes. I have been underneath the house and the pipes are not knocking against the joist or any thing. The knock happens when I turn the hot water on. It also happens at times with the water off and I am walking across the kitchen floor. The hot water heater was replaced recently and it did before and after the heater replacement. Any ideas? Thanks.
Sounds like you may have water hammer. Back in the 60's we were required to to put shock absorbers,(air chambers) on all bath lavatorys and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is importent.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will begain to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps and thank you for rating my reply. TOM