Hum sound from Spa breaker box

If it's a rain tight breaker box with a double pole gfci breaker, you've likely got a bad breaker. If it's a GFCI protected contactor type spa disconnect, like the one in the following link, it's likely a buzzing contactor, which could be from dirt, or corrosion, and probably needs to be replaced :

Thanks! It does look like the one in the link. This is only a couple of years old. Am I really going to have to replace this entire $140 thing this often? Or is it just a smaller, read 'cheaper', part I can replace?

Bryan

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The contactor in the unit,(the thing that's buzzing), has an electromagnetic coil that pulls the contacts together. If it doesn't seat properly, it will buzz. Sometimes it's a matter of insect debris, or oxidation causing this. Kill the power to the unit, open it up, look at it and see if you can clean it up and stop the buzz. A little WD-40 may help, but don't get it on the electrical contacts