We have just moved into a new home that has metal air returns. Apparently the metal air returns create harmonics. How can this be stopped.

Is this a new home or new to you?. If a new home have installer rework. If new to you, find out where area is that is vibrating and start applying pressure to the ductwork. You can use a rubber mallet to slightly reshape. If it is in the return grills you can usually bend the vanes slightly. If in the walls, your task is bigger and more difficult.
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It is a new home. What do you mean by the installer reworking?

Since this is a new home, you can call back the builder and tell him the issue. Have him get the AC ductwork installer fix the issue, it is under warranty.