Without hearing it I can't say for sure, but do you brake intermittently or continuously while going downhill?

If the latter, it'll likely be a caliper & disk getting hot and expanding, and rubbing, which can make an almighty racket - and then when they cool again they contract, meaning that a cold inspection would reveal little.

Alternately it's a wheel bearing on its way out, but that's unlikely on a new vehicle.

Does it get any better or worse when you apply steering, or not change at all?