Probably 120 Hz noise from the power supply transformer that is mounted on the large circuit board (that is mounted right behind the front face of the EVSE). If so, you should always hear it (when the EVSE has power) by putting your ear right up against the front of the EVSE.
Assuming it is the transformer hum, some mechanical coupling between the circuit board and the front face probably causes the front face to act as a "drum", resonating and amplifying the "hum".
Strange that the noise would be intermittent unless you are occasionally flexing the front cover, by touching or pressing it, or by pressing a button.
Perhaps one of the heavy e-hose wires connected inside was "squashed" back inside when the front face was mounted, so a little flexing of the e-hose causes that "strong" wire to press harder against the backside of the circuit board, flexing it toward the front-face "drum".
Wiggle the e-hose near its connection point to the EVSE and see if you can make the "loud" noise come and go.
The louder noise would annoy me, so I would have to "fix" it.
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