2003 Ford Expedition loud metal on metal sound comes and goes

This sound was similar to the sound produced when closing gas cap and it clicks even though it’s already closed, but MUCH MUCH LOUDER. It sounded ominous, and seemed to be coming from left rear side, like it was related to wheels somehow. First it happened when moving into a left lane driving 10-15mph, then stopped, then started when braking, then stopped, then started up again when accelerating. Thought I could make it to mechanic while driving slowly, but then it started up again when just driving straight. Towed to mechanic, who could not replicate the noise when driving it himself. Also put it up on the rack and only found wheel bearing issue which would NOT have made that kind of noise. At first I thought something had gotten caught up in the wheel well but it was definitely a loud METAL on METAL sound, not like a tree branch or something. What should we be investigating to find the cause of this sound?

Not necessarily true. If the disk or drum shifts far enough because of the failing bearing it can make a pretty significant noise.

The bottom line is we can’t hear it, we can’t see it and are not right there with the vehicle like the mechanic is. Other than my previous comment, I’d suggest you keep looking!