Loud metal sound when not pedaling
If I understand well this noise only happens when the wheel is rolling and you are not paddling. If so, then the most obvious place to look first is the freewheel. It is in the back wheel axle.

You can try to reproduce the noize by lifting the back of the bike and spinning up the back wheel with the pedal to a high rotation.   

One cause could be the lack of grease/lubrication which can make the freewheel noisier.