I have a new lennox 2 stage furnace. The furnace works fine except that the 24 volt transformer hums so loud that you can hear it through the ductwork when the furnace is off. I have tried to isolate the transformer but have not been successful. Any advice?

I'd have to see the unit to decide. But I would relocate it with a rubber pad between it and the metal you screw it to.

They aren't that expensive. You have a good chance of the next one being much quieter. If it is held together by bolts, you could try tightening the nuts.