Loud Hum In GFCI Disconnect

I have an SBSG brand 50-amp GFCI disconnect for my spa. It has a loud hum. I have talked to the factory, and they suggested loosening the screws to the contactor, then retightening. That helped for a little while, but now it is back worse than ever. Being mounted outside the kitchen wall, it resonates throughout the kitchen, and is very annoying. Is it possible to use some nylon or teflon washers under the contactor screws without violating code?
Dick Remove 33 from address

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