I seem to remember that my first Mk3 made a bit of a sizzing sound, and I think it was down to me pratting about with the blue pot. I pratted about with it a bit more and it went away.
Take the gun out of the stock and look for a blue square thing on the circuit board towards the back - it's about 1cm x 1cm with a white plastic adjuster screw in the middle of it. If your mk3 has got this then I think it's a model B or early model C board - anyhoo.. turning that white adjuster screw will set the power and 'may' make the buzzing noise go away.
p.s. I'm no expert