I would also say that mine never fully goes away, it's just quieter when warm. Maybe you have sensitive ears!You are right most of the guys here saying it goes when engine warms up, but mine does not, stays that way even if I drive for long time.?
This car makes some weird sounds, like a more uncivilized machine. I say roll with it and learnt o appreciate the odd machinations of the Mustang. It is not like other cars that is for sure lol.