Pipes knocking

Something called a Water Hammer suppressor would fix it.

Near the valve solder in an upside-down "T." Solder a 2' capped pipe to the T. This will absorb the shock waves.

Does it do it when you turn the water off or as the water is running.

In the first case it is likely water hammer and there are several solutions depending on the situation.

Since you specified hot water, I am going to guess that you have air pocket type anti-hammer devices that have become saturated. That is easy to fix.

The easy way first. Start by turning off the heat to the water heater. then turn the water off at the house shut off valve and then turn on all the faucets in the house until they stop running (leave them all open to drain the pipes. Then turn the water supply back on and start turning them all off once they are getting good water flow.

Now try to make it hammer. Usually that will do it.

I have been on a few jobs where plumbers have been called in to address a problem with banging noises from the pipes. The end result was that the problem is a result of expansion of the pipes when hot water is run. Since you mentioned that you only experienced the problem when the hot water is run, perhaps this is your problem. Does the banging stop after the water has run for a few minutes?

My experience is different than your other replies. Don't know if it is more appropriate or not. My hot water pipes knocked. I found that over the past 25 years they had broken free of nearly everything that was holding up. When I reattached them with loose plastic hangers they quieted right down. (I first reattached them with metal hangers and the noise got much worse...)

I also have occasional banging of the pipes sound. It typically only happens when I run the shower (hot water) and mostly in the winter (i'm in idaho so sub-freezing temps).

John