I experience the same.
I also experience this if I accelerate suddenly. For example, if the car is coasting at 25 mph and I suddenly push the gas pedal hard, I can hear the noise.
Something has to be loose in the drive train. The flex disk looked OK. So I think, in my case, it is the center bearing. It could also be the differential. I checked the differential mounts and they looked OK as well.
The rotational free play I mentioned earlier can also be felt at the rear drive shaft.
I ran out of space to see if the play is between the front and rear drive shaft. The car has to be on a lift foe me to try that.