Strange intermittent noise underneath vehicle

I purchased a 2013 Explorer XLT last July and 3 months later, a strange
popping/rattling noise began one day...noise sounded like driving over fresh
asphalt paved highway with popping noise underneath. Shortly thereafter,
the popping noise stopped and a new noise, like a rattle, flapping, scraping
sound began and it continues even today. This noise is intermittent, with the
noise heard on driver's side, then it moves around the vehicle to rear and
back again to driver's side. It sounds almost like a metal scraping sound as
in the frame but 4 different garages and dealerships can't find anything
wrong.