I didn't realize that your walls were closed. You could do it from the bedroom side, you just have to do a more careful patch job since you will be able to see the drywall. Insulation isn't typcially added around pipes, so it isn't something a contractor would automatically do. We chose it put insulation between all of our rooms to help reduce noise. Also, you could access the pipes from the inside of your cabinet if you cut the back of the out. The thought does make me cringe, but there again, how often does anyone get down on the floor and look at the back /inside of the cabinet?

I just saw this on Ask This Old House, The shut off was only open 25%, This valve should be 100% open or closed. Brian

On the second floor, the chase is inside a built-in pantry cabinet in the room that is our kitchen. We hear the banging there also, but it doesn’t bother us because we don’t sleep there :) My tenant says he does not hear the noise on his floor (and the chase is in his bedroom), but I can certainly hear it in the cellar below him. The chase on his floor is bigger than on the top floor because the window placement didn’t cause the same space restriction, so perhaps the chase on that floor has better sound insulation or maybe the flue pipe is not as tightly squeezed into the chase on that floor. In the cellar, where the flue meets the boiler, the banging doesn’t sound quite as loud to me as it does on the top floor, but it’s hard to tell for sure—it’s not an apples to apples comparison, because down there it’s an exposed pipe not encased inside a chase. It’s attached to the boiler (and two water heaters) via a 90 degree elbow, and screwed to the wood joists with metal clips in a couple of places (see attached photos) — I’m not sure how much flexibility of movement that allows.

I would call an experienced, accredited hydronic heating specialist to diagnose and identify the source of the problem and propose a sensible course of action from there. Opening the walls to see what’s there might not be necessary or sensible since what’s there has always been there with no problem. It’s quite possible the source of the noise has its origins elsewhere in the system and you might not have to open the walls at all. You don’t know but it would be good to find out before you find out that you didn’t have to. IMPO SR