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Tell me if the system is cooling and heating OK or not... and about how old it is. And if the humming is steady and constant or intermittent or changes pitch.
We can go from there.
Thanks, we should do some basic maintenance first.
Tell me if you have ever cleaned the fin and tube condenser coil that surrounds the outside of the unit.
Noise testing is done by feeling for the vibration with your hands, you can generally feel the source, if that fails you can use a mechanics stethoscope ($25 or so at an auto parts store.)
We keep going until we get the cause sorted out.
That video is on the topic of cleaning the condenser coil.
Remove the covers, and use tape to hold the blower door switch closed.
use a heavy wooden cooking spoon to press on various parts of the inside unit until you can feel the vibration, be careful not to poke the spoon into the spinning blower wheel.
And tell me how far you have to get away from the unit before you can barely hear the humm.
Note: The 24 volt control transformers tend to humm gently, thats normal,
Those are pictures of typical 24 volt transformers.