Wow! Thanks for all the welcomes and suggestions. I shot a video with the noise starting at about 22 seconds and then again at 43 seconds but it's really hard to hear with the wind. Maybe some trained ears ;-)
I tried the emergency brake suggestion and it doesn't seem to be the back brakes. The only thing I can add is that it only does it after it's well warmed up (10-15 minutes of hill climb) and it's definitely less noticeable at 35 degrees (this morning) compared to 95 degrees (last month).
It sounds like the most likely explanation is that it's just normal sound of the slop in the gears since several people said theirs always did it and it's not a problem.