I'd probably go with the bird theory myself, but let's think about it for a second...lets say it was a bad guy. What do you suppose his plan was by rapping on the window? I'd say dollars to donuts it would be to get you to come running outside.
When it comes to stuff like this, stay inside where you have all the advantages, and call 911. Let's say you get the cops out there, and they look around and don't find anything...they aren't going to write you an invoice or something. "We didn't find anything" "Wow, what a load off my mind. You guys are the best" That's all that happens.
I'll tell you a true story. I used to listen to the scanner quite a bit back in the early 80's. I heard some cops dispatched to a call at an older single womans house, where she claimed a man had been trying to kick her door in at about 3:00 AM. You could just hear the "Yeah, right" in the dispatcher and cops voices. The cops (wisely) got a few extra cars out there, and searched the perimeter of this womans house, only to find bootprints all over her front ddor, and the same boot prints in the dirt in her yard- it was exactly what she thought it was. You could hear the mood change on the cops as they widened the search to the surrounding neighborhood. I found it pretty chilling. What was he after? Should she have opened the door to see who it was?
Stay inside. Call 911. Don't search or clear solo.