Buzzing is usally AC line noise leaking into an un-shielded cable somewhere or a floating ground. If the AC plug is reversible, try that. Sometimes the buzz is from an attached device. Make sure all audio cables are plugged in tightly at each end and routed away from or at right angles to any power cords and away from other sources of strong magnetic fields like TV's.

Disconnect the input(s) and see if it hums in the absence of an audio source. A bad audio cable shield or unwisely-routed audio cables will allow entrance of unwanted signals from external power sources, magnetic fields, even dimmer-controlled track lights.
I've also seen variable track lights induce noise but it's usually minor.

Try rotating the power cord in the wall outlet.

If it still buzzes with no cables or anything external attached there's an internal problem.