It isn’t due to rust. That was just pure BS.
I think the first thing I’d do is elevate the wheel and turn it by hand to see if it also makes the noise when not loaded. The car will then need to go on a lift, have the wheel removed, and be examined, including having the hub turned by hand without the wheel on. By that point the mechanic should have pretty well isolated the source of the noise.
It may be as simple as a bent dust shield behind the disc, or a loose/hanging wheel speed sensor (do you have any warning light?), or you could even have a fracture in the wheel that’s allowing it to move when the wheel rolls like the two sides of an earthquake fault… and it needs to be looked at for that if the noise goes away when the wheel is unloaded (elevated), because a fracture of that nature could fail suddenly. A fracture of this nature could also be verified by moving the wheel to another corner and seeing if the noise follows the wheel.