Hearing knocking noise when hot water is running...

i think it means that there is air in the lines.

Water hammer would usually occur when you turn the flow off. Those extensions you see on some waterlines that come to a dead end are there to cushion the water flow against the trapped air in the extension. Over time that extension can loose the air cushion which would result in water hammer.

Water Hammer Wiki

Note the air chamber in the photo. That provides the cushion. Shut the water off to the house, drain the system, then turn the water on. Might fix you up.

Not sure if it is air since it is a single knock every 4 seconds...doesn't air make repeated knocks, like a machine gun going off?

Pigskin, it only happens when the water is running...thoughts?

Think this one might nail it Chicken...

Water pipe knock

Date: 3/19/2005
Subject: Water Pipe Knock

Question
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.

Answer
Hi Elton,
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

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Anyhow, just wanted to bounce this off my OT brothers and sisters before I get a plumber out here.

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Did you check the fetzer valve?

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I think it has to do with the exhaust pipe which runs up the wall. Possibly the hot water is causing expanding or hot gases to make the knocking.

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Are you planning to have this fixed? I would be curious to know if this solved the issue...

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I just determined that I only hear it when the main shower in running...you can hear it in the floor of the closet that is adjacent to the shower.

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It could just be expansion of the line then. As the hot water is running the pipe expands just a bit.

"When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight." LINK

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To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight."

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I was afraid of that...