I can hear what sounds like a chewing sound in one of my wall voids. At first I thought it was a mouse or rat but the sound seems to stay in the same area all the time. When I go outside and look around the only thing I see are large black ants going up under the siding. Could they be doing something in there I might be hearing?
The ant you're describing is probably a carpenter ant. These are large, black ants that nest in wood. They have strong chewing mouth parts and will readily gnaw through wood. This gnawing can get quite loud and I'm pretty sure it's what you're hearing.
Since carpenter ants can cause a lot of damage, I suggest you either treat the home as described in our Carpenter Ant article or get a service company to do the treatment for you. The gnawing is usually the sound of a new infestation and though it will probably subside in a few weeks, the ants will still be present which will only lead to further problems.