Hooray, I solved the problem! Turns out it wasn't the blades themselves, but rather one or two of the arms on one side were not fully extended.
Diagnosed the issue by taking off all props and putting each back on at a time. It was just one one side when both props on that side were put on that...
Thanks for the suggestions. I flew the drone again for just a few min and I could still hear the noise. It was obious that one or more prop was "hitting something". Remarkably, there was no apparent effect on flight, but after landing and turning off the motors, It was clear that something...
I crashed this afternoon and damaged one of the props. During the process of replacing it I noticed it looked different than the others.
Duh, there are two pairs of props. Not all 4 are the same. Helps to read the manual!
So after getting the right props on the corresponding motors, I turned...