"Black-Capped Chickadees are bold and inquisitive and are usually the first to find a new feeder. They appreciate a reliable food source and have been known to tap on windows if the feeder runs low."
The Chickadee may be trying to get your attention or most likely get in. A good solution is to check the feeder for daily fillings, or look for something that is irritating the bird. For example, a large squirrel was bombarding our feeder, and the birds got angry at him. They perched in the pear trees and would chirp loudly. We put out safflower seeds and not as many squirrels came.