My husband replaced a cartridge in a shower. He shut off all the water to the house. The cartridge is replaced. Now, when we turn the hot water on, we get a hammering noise. We have turned off the water, drained the system, and turned it back on 12 times and it still exists.
We also have a water recirculator on our hot water heater for instant hot water. The noise appears to come from this.
First, make sure the valves leadong to the shower are fully opened. You may have a valve in the basement to shut off the water to the risers for the second floor (assuming this shower is on the second floor), valves directly behind the shower, and, if it is a Moen shower faucet there may be internal valves inside the faucet. To check for the internal valves, remove the large round coverplate under the handle, and look in the hole for two "screws" that can be turned with a straight blade (not phillips) screwdriver and make sure they are fully open. If that doesn't fix the problem you can buy or make hammer arrestors and solder them on to the hot water pipe. Basically they are simply a 6 inch piece of larger diameter copper pipe with a cap on it. If your existing supply pipe is 1/2" you would use 3/4" pipe for the arrestor. This creates a trapped column of air that will act as a cushion as the water goes on and off. Google "water hammer arrestor" for more information.