Welcome to genvibe
Hard to say without hearing it, and hard to isolate when it only happens when the car is moving (accelerating in gear). Nice job trying it without the serpentine belt, that eliminates most of the usual suspects, and having it relate to engine speed and not vehicle speed eliminates a lot of others.
Maybe its something like an exhaust leak, or a loose heat shield.
If you can recreate the noise with the car in gear, one foot hard on the brake so the car doesn't move, and then rev the engine, maybe you can have someone you trust do that while you look in the open hood (from the side of the vehicle
) to try to source the noise