Thank you to everyone for your suggestions! I will start with checking out the seatpost (since I can't seem to duplicate the noise when I'm off the bike) and look for other things that may have loosened up in the breaking-in process.

It appears to be related to pedaling activity. With the shift of weight that occurs in the saddle during pedaling...that makes sense.

Oakleaf <LOL>-I'm pretty sure it's not my knee; my other bikes do not make this noise.