Just a guess...but a reasonable guess at that: dry or dirty BB threads. Manufacturers don't always adequately lubricate the threads; and some riding conditions can carry fine grit into the threads. Did you ride (or transport the bike) in wet conditions? Removing the BB cups, cleaning the threads and regreasing & reassembling could resolve the issue. Will require a couple tools to remove the crank and BB...and a bit of spare time.

Of course, there are a multitude of sources for strange and undesirable bike noises, and my guess is made partially on your statement that you've eliminated other possible sources outside of the drivetrain. Since you stated you fixate on such things, have you acquainted yourself with Sheldon Brown's writings on such?