Burrowing mammal is most likely. If the hole is less than 3 inches (7.5 cm), then the following two are the most likely candidates.
My first thought would be rats. Rats build quite extensive burrows to nest and to enter dwellings. I had a look at the mammals of Pennsylvania and the Norway rat (Rattus norvegicus) is the most likely candidate. As you probably know, rats are very common where-ever there are humans and are found in both rural and urban environments. Rats are nocturnal/crepuscular usually and quite secretive, so you may or may not see them around. Rats tend to leave the dug earth at the entrance to the burrow, which the next candidate doesn't do.
Another possibility is the chipmunk, which are also common in much of the USA. They build burrows for living as well as hibernation. They tend to have multiple entrances with a nest in the middle. Their entrances tend to be very clean as they carry the dirt away in their cheek pouches.