The noise is due to the dilatation/contraction of the hot water pipe rubbing against the wood studs — the noise is actually coming from the wood stud. There’s nothing “structurally” wrong with that.
Like bugs, says, it’s easy to open the wall and fix that problem if you want to (probably about $500). Otherwise, you could turn off the hot water recirculating pump but that would cause your not being able to get “instant” hot water at each application.
Interesting how hot water recirculating system saves water but wastes a lot of energy.
