The motor sounds like a regular Dyson motor, no odd grinding or moaning. The flutter is overlaid on that (i.e. there are no skips or changes in the motor sound during operation), which sounds pretty much like a bird trying to fly.
I noticed that the vacuum was hissing during use, which set me on a witch hunt to find where the leak was. When I was poking around, that's when I heard a faint fluttering. When I put my hand on the wand, the motor strained, the fluttering was louder and the relief valve never came on (however, if I poked it it would open and stay open). I think there is a separate issue with bleeding somewhere which I need to address, but the motor sound is concerning me a bit more. I still works great even if there is a bleed.