New Legacy with a rattle in the front driver's side wheel area.

The clue is it goes away during braking. Most commonly a brake pad is loose and rattling. There could be a broken or missing anti-rattle clip or a pad's mounting fingers could be deformed slightly. A loose pad is not a safety issue but it can be annoying.

Normally your mechanic would be able to find this by removing the wheel, then pounding in the area with a rubber hammer, but to add to the misery, most manufacturers are using "low drag" brake calipers to improve fuel mileage. Those let the pistons in the calipers release more, and that allows a loose pad to rattle more than normal. When your mechanic drives the car into his work bay, the last thing he does is apply the brakes so he doesn't go through the wall! That pedal application, along with no bouncing down the road, keeps the pistons out to where they hold a little pressure on the pads. Now those pads won't rattle when searching with the rubber hammer. That's why they often can't find the cause of the noise.

What I would do in a case like this is, with the wheel off, use a flat-blade screwdriver to pry the piston in a little, THEN go after it with the rubber hammer. It's pretty likely he will be able to make the pad rattle if it is loose.