a few years ago I installed this autocirc pump in my back bathroom under the sink that pumps the hot water into the cold line based on a timer so hot water is available quickly.

Whenever the cold water is turned on now I hear a loud knocking. I am guessing it is the pipes as I don't hear any knocking when the pump is active - it sounds smooth. Shutting off the autocirc makes the noise go away during cold water turn on also.

What is the typical solution to this ... my guess is that the cold water pipe is loose behind the wall but before I tear into the drywall to see I figured I would ask if anyone had a similar experience and how to resolve with the least amount of pain.