McIntosh MA5200 hum

Hi, I bought a three month old Mc MA5200 from a reputable dealer. Brought it home plugged it in and immediately heard a hum. Not through speakers but from amp. Tried plugging it in everywhere in house, same result. No hum comes through speakers but can here hum from amp between songs from 10 ft away. Took back to dealer next day, plugged into his wall direct, no hum. Brought it home and same hum produced. I have lots of other amps and made sure that none of them are humming. Very perplexed. Does this amp not like my AC? I do not use a power conditioner. Any thoughts on what to do next would be appreciated. I could probably live with hum but why should I? Worried that amp might need repair. But again no hum at all at dealer. Thanks for your thoughts.