Roughly twenty species of cicada occur in Maryland, so populations can fluctuate. They make unique calls, and can also call at different times of day. In the evening, calling insects tend to be crickets and katydids (often the loudest), for which there are also several species possibilities. Some websites have recordings you can listen to of the calls of various species so you can compare them and try to ID the caller. Here is one example that might be useful (not all of its listings occur in Maryland, but they do include range maps): Songs of Insects.

Miri