Motor bolts, my reaction 500 pro made the same noise. I regreased my seat and stripped it 3 times before I read somewhere about motor bolts.
Mine were not even that loose, less than quarter of a turn but that's enough to make that awful noise. Mine was on the downstroke with the left pedal.
If you do take the covers off to check them, tighten the T-30 torque screws as well, mine were also loose.
Might be your seat, but that sound in the video brought back some horrible memories
Edit*** Check your cassette as well because that was the next thing to come loose the very next ride after fixing the creaking noise. I checked everything whilst tightening the cassette and most was as it should be.