re: Hearing knocking noise when hot water is running...

If you want to do it yourself, ask someone who knows, perhaps in your local Home Depot or Lowe's, in the plumbing section. They should know.

There is a very good possibility that when the home was built they put wood shims in the holes to hold the water lines tight. Over time those shims can fall out which will cause the pipe to knock in the hole. Best way to solve that problem is to figure out where the problem is, cut a hole in the sheet rock then use spray foam in the hole. It will secure the line, and stop it from ever happening again. This is why when I build a home I make sure there are no wooden shims in the holes that copper runs in, only spray foam. Also a reason why I do not run copper anymore.

I'll come supervise Fred fixing it if you buy the beer.

Note sure if I want Fred's vulgar mouth around my kids...

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There is a very good possibility that when the home was built they put wood shims in the holes to hold the water lines tight. Over time those shims can fall out which will cause the pipe to knock in the hole

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This could likely be the problem. If you can get to it from the attic, sometimes going through the top plates could be your problem. Shoot some foam. If its in a wall, and you can locate the area, cut a square of sheetrock out. Remember to cut halfway the thickness of the stud, this will allow you to nail it back. If its on a sink wall, use the mirror to cover up the opening. A friend of mine had a problem a little different from this. His wall was cold, thats right the bathroom wall above his sink was very cool. He had removed the mirror and cut out a opening before he called me. I get over there and hes pulling celulose insulation out of the wall. When I get there and I find cold wet insulation, I tell him, I think its coming from the attic. I get up in the attic and the condensate drain from the A/C unit wasn't glued when the house was built. The mirror covered the hole and glue took care of his problems.

Chicken, if you run hot water from any of your faucets, shower, etc.

this is what's happening.

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Sometimes the knocking radiates along hot water pipes from a sediment-laden water heater that is in need of a good flushing

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Mine does it too. Especially if it's a gas WH. Tom Tynan dsicussed it on his home show a few weekends ago too.

Also, do you have an upstairs WH and a downstairs WH? You probably do in that mansion you live in now.