Look closely at the first few seconds of the video that shows the long run on the V belt at the back of the deck. Use your cursor and slow and stop the video while watching the long run on the V belt Something strange going on with the belt on the long run, appears maybe the belt wants to twist or ??
I think I would change the belt. Check the idler and it spring, but appears the vibration maybe starts due to the belt on the long run at the back of the deck???
Be careful around that beast and leave the blades off while it vibrating and you are out of the seat watching, etc.