I don't live in an old house with the same construction but I added an access panel in my master bath closet when I needed to change the fixtures for the tub/shower. I didn't even have a door until they added it during my current remodel :-) as I just put a piece of cardboard there to close up the hole.

I never heard noise coming from the access panel in excess of what I would have heard through the door. It didn't seem to change the acoustics at all. But my construction is different as it's a 1965 build with sheetrock in the closet. The old tub/shower was one of those all in one acrylic things with walls and I changed it to a tile wall but I don't think that would change the acoustics since the access panel replaced sheet rock in my closet so only change is that instead of sheet rock I now have a door.

I have the same wall construction in my 1940 house - plaster over rock lathe (which looks like sheetrock). Careful work with the oscillating tool will generate some dust, but if you are patient you can get nice clean cuts through all the layers.

I have plumbing access panels in three places, and have not noticed any increase in plumbing noise. If you do hear noise, you could stuff batt insulation in through the panel opening, and just pull it out when you need access.

Take a look at the metal access panels available from HD and Lowes. The biggest one that will fit easily is about 14" x 14", so it is designed to fit between standard spaced studs. You can cut out the opening, slip in the panel, and put screws through the flange on each side into the studs. Its a fast install once you get the opening cut. I painted mine the same color as the wall, so it blends in nicely.

They also make larger metal access panels, but you would have to frame an opening for them. I did do that in my basement where the gas meter is hidden behind a much larger panel.

Bruce

OK thanks very much for the responses, it makes me feel more confident. I'll give it a try to make a smaller opening and see what's there. Anxious to see how hard it is to cut.

You could check on the remodel forum too for info on your cuts and walls.

This is what I found. Another wall doesn't seem to have lath (and maybe I'm wrong about that wall), but these pipes are sandwiched between two lath and plaster walls. The horizontal hot/cold pipe is just over the 2x4 that's been used as a support. Awaiting some advice from a plumbing forum to see what the best way is to proceed.

What is the surface on the other side of this wall? It doesn't really look like plaster and wood lathe I've seen, so I wonder if its cement backer for a tile surface on the other side. Many of these houses had bathroom walls with tile up to about 4 feet.

The horizontal pipe running through the floor cement area is probably running through the bed of cement they put under the floor tile. In that time period they did not put tile over a simple subfloor. They built a tray lower than the the floor level, filled it with cement, and then put the tile on the cement. It really makes it hard to access those horizontal pipes.

Bruce

It's a tiled shower wall, maybe original to 1939. The bottom of this square hole is 30" above the floor, so it's not floor piping. That stuff is the plaster (I think), which has pushed over the pipes in some areas. The second picture is misleading because the camera is very close to the 2x4, so the top edge of it (bottom right in the 2nd photo in my post above) looks wider than it is. The lath-and-plaster looking stuff is the same as what I cut through to make the hole. I took some close-ups of the "plaster," or whatever it is, which I kept in a little sample bag because it's so interesting.