Why this happens

Family Handyman says banging can come from pipes that are not properly supported or from loose joints, including from expansion and contraction as temperatures change. Bob Vila says that if the U-shaped straps holding supply lines are loose or missing, the pipes can knock around against framing and make noise. Oatey says to push gently on visible pipes, and that a pipe longer than 16 inches that moves easily should be secured.

How to fix it

If you smell gas at any time, leave the house and call 911 and your gas utility from outside. The Pennsylvania PUC says not to turn lights on or off or make calls from inside.
Work only on water pipes you can see in a basement, crawl space, garage or utility room. Leave gas pipes alone. Do not open walls or ceilings.
With no water running and the pipes cool, push gently on the visible pipes. A pipe that moves easily is a suspect.
Tighten loose straps with a screwdriver, or add straps, as Bob Vila describes. Padded straps add vibration reduction. Foam pipe insulation, slit down its length, can cushion a pipe that knocks. Keep it away from the water heater's vent pipe and any other gas appliance. If a knocking pipe is that close, call a plumber.
Bob Vila says never to use galvanized or steel straps on copper pipe, because the mix of metals leads to leaks. Copper straps are the better choice.
Do not loosen, bend or cut any pipe or fitting. If the noise is inside a wall or ceiling you cannot reach, or tightening does not help, call a plumber.
If the bang also comes right after a valve or appliance shuts, it may be water hammer, which tightening straps may not fix. Family Handyman says the best cure is a water hammer arrester, and a plumber can advise on one.

When to stop. If you see a drip, a damp spot or a stain on or near a pipe, shut off the main water valve by hand and call a plumber. Never force a stuck valve.

If this does not fix it, tell the plumber

Where the noise seems to come from, and whether it is hot water, cold water or both.
What you tightened or cushioned, and whether it changed the noise.
Whether the bang also happens when a valve or appliance shuts off.

Build the full list: what to tell the plumber.

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References (every fact above comes from one of these pages)

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