How can I stop our duct work "pinging" at night?
Sometimes ductwork under pressure and heat can experience a sudden deformation commonly referred to as oil-canning.  This is where large rectangular pieces of box ducts have stiffening ribs formed into them to prevent deflection under pressure.
If a particular panel is installed in just the right (or wrong) way, it can temporarily flex when it warms up.  It's usually coincident with a loud ping as described.  It generally repeats when the heating cycle ends, it cools down and it returns to its original shape.  If you can find the specific duct portion that's doing it, you can stop it by gluing a damping plate (piece of plywood or steel) to the outside face of the offending duct panel.