I live in a 2 storey house in London, Ont, Canada. Upstairs we have 1 bathroom. About 20 minutes after the water is used in the tub or sink I hear a load banging sound in the wall. I have exposed the pipes in the basement (pic attached) and the main level (pic attached, just horizontal instead of vertical) and I see that the hot and cold water copper pipes run along a heating vent also heading upstairs. The drain is also within this same area. I have tried to bend the heating vents that come in contact with the copper pipes but the sound persists. I have attached a file with the sound. Any idea what this is and how to stop it from happening?

I listened to the sound and it doesn't sound like air hammer to me, It sounds more like expansion and contraction.
Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. 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. Most people when they learn what causes it just elect to live with it. Good luck and thank you for rating my answer. Tom

I have exposed the pipes as you can see by the pictures. This sound only happens when the upstairs bathroom is used so I assume it stems from the hot water pipe heading upstairs. The first pic shows the hot water going up from the basement to the main level. I can see this pipe spanning the length of the house from the hot water tank. It is strapped to several floor joists in the basement ceiling. It then heads up through a wall in the main level (2nd pic). I have since wrapped this pipe and then strapped it. Once it gets up into the ceiling it is drilled through the joists so I assume this is not where the problem lies. How can I determine which strap (if I assume it is one in the basement ceiling) is causing the problem? What then do I do (it has to be strapped, right?).

Your noise is caused by vibration. If this happened to me I'd get the pipe knocking and feel along the pipe for the strongest vibration and then shim it tightly to the joist. Good luck, Tom