240v charger was buzzing like a transformer...

Went out to my garage and heard a buzzing sound coming from my Sun Country (Clipper Creek) 240v charger. It sounded very much like a transformer buzz you hear from a large transformer, not real loud but loud enough to hear it in the hallway beside the attached garage. I unplugged the charger from the car, waited a few seconds, plugged the charger back into the car and all is normal again.

Has anyone else experienced this and could this be a sign of imminent failure? The charger is about 3 years old and is used daily. I guess time will tell but I have never heard any sounds from the unit other than when the relay operates to start the high voltage charging.