[Pimptastic]

It sounds like a grounding issue. Do you recall if the flickering they did previously pulsed about as fast as the buzzing you're hearing? I suspect the capacitors didn't fix the initial problem, they merely corrected the light output issue. The relay could still be "buzzing" on and off rapidly because of a bad ground or not enough voltage going to the ballast to maintain the light source. The capacitor is simply storing and releasing power to keep the lights on when the relay opens and closes as it does. I would check grounds and connections.