When Goodman heater is turned off is gives off this very loud banging noise. It is only about 4-5 years old.

No need to post twice, we are looking into your problem. Make sure the filter is clean. Sometimes positive pressure is released when the heater turns off and the return flexes back to it's original shape. It could also be the plenum expanding and contracting during the heat cycle. Cycle the heater and see if you can pinpoint the area where the noise is coming from and get back to us. .