Really doesn’t sound wheel bearing like and it also doesn’t look like the sound is a one to one rotation with the wheel... it sounds faster based on the video. The fact that it only happens with the weight of the truck on it should tell us something too. You may have to drive that side of the truck up on a ramp (so everything is more compressed) and slide under there and inspect the entire length of the rotating assembly - sounds very much like something is rubbing. As EODGuy said, almost sounds metallic, like sheet metal scraping.

Did your mechanic thoroughly inspect everything for scrape marks? I would think if it is external that something has to have a mark or ten on it.