I hear critters in the walls of my house. These are primarily outside walls, not interior walls. They are in an area with high ceilings and little access to the space under the roof supports. I hear them up high in the walls and they aren't always in the same place. I don't see how they could get in the areas from the outside, so I assume they are entering through the crawlspace under the house. I don't really want to cut holes in the walls, but there are a few cabinets attached to the walls where I could make cuts that would then be easy to hide. Where do I start dealing with this issue?

I'd start by carefully looking around your house for entrances. Pay close attention to corners on the bottom and top. Dormer tie ins, eyebrows, ridge vents and gable vents are the common spots for entrances up top. Also seal up pass through a for wires. Especially around the ac line power line. Look for rubs where they are coming in and out. Do you have flying squirrels in your area?

yes

What time of day or night do you hear them?

Is the noise a scratching? Running? or Both?

Is the sound going along the wall or up and down the wall?

Is it every day or off and on?

I usually hear them in the early night, after dark, starting about a half an hour after dark. Hear both scratching and running. It is hard to tell about direction. often is sounds like the noise is moving through the crawl space above the ceiling and below the roof. Sometimes scratching in the wall, usually up high on the wall. I looked at the eaves and climbed on the roof today and saw no holes anywhere.

most people don't in my experiences,,,,, I know nothing about S.C. but sounds like squirrels..........

Hiring a pro might be cheaper then cutting holes in the wall

Or you could hire a pro who cuts holes in the wall. That would really be expensive.

I don't know. I'll put all sorts of holes in walls for free. It's fixing the holes that gets expensive.

Next time you hear it bang on the wall. Flyers will run, mice will freeze. Not a 100% test but works most of the time. I am guessing flyers. If they run try to track the direction, they typically run for an exit.

Reds run too. They'll squeeze through a pretty small hole too... Or open it up large enough for them to fit.

x2 RF I don't think SC has the red squirrel. May be wrong. grey squirrel should be hearing in the morning.

lol,,,,, we do a lot of smoke tests in Arizona,,,, and I charge to stand there looking for it (Norway rats)

No reds here. I trimmed some branches that were hanging over the roof and swept off the roof and the noises have stopped, it seems.

Maybe was the branches rubbing against the roof when the wind blew.

What is a smoke test for rats?

Vinke lights up a cigarette and if a rat doesn't try to sponge one, he figures there are none and moves on.