Hammering Noise - Water Recirculator

Hi -

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.

Any ideas?

Thanks.