Select all interior faces of a volume

I have a very noisy model and need to extract some parts. Is there a way to select all faces when zoomed inside a closed volume so that it can be copied? I’m new to 3d editing so there might be something simple I have missed.

Screenshot would help to know what you’re talking about and give more precise answers. The term “interior faces” means a non-manifold mesh error and there is a way to select those in select menu -> interior faces. Also select menu -> non-manifold would work to find them.

If you just need to access them, you could change to wireframe shading mode (z), select one or more mesh elements and press ctrl+L to select rest of the mesh part that is linked to those. If you have lots of geometry enclosed with outer surface but not connected to it, you could select the outer shell similar way and then press ctrl+i to invert the selection. Hovering your mouse cursor over the mesh part and pressing L also works for selecting.
If they are connected, you could hide the connection parts and then select like mentioned. Or you could do a partial selection and ctrl+numpad_+ to grow the selection until you have the desired part selected.

Clipping border (alt+b, same to disable) helps to see just part of the object(s). When you press it, it gives you a box selection tool. Draw a box on the parts you want to see and it hides the rest.

It is a model constructed from xrays. There are different layers of bone and skin and I need to get only the brain. It is probably connected on some points to the other layers. Its to many faces to manually mark them.