Problem with my Nad 304

Hi guys
Just bought a Nad 304 and soon found out there's a slight hum on the left channel. Tried the other set of speaker terminals, changed the cable, swapped the connections, tried a different set of speakers, cleaned the connection terminals, it just keeps doing it. It becomes more evident on higher volume.
I can live with it but I'd rather have it fixed. It seems like a problem within the amp. Anybody has an idea what I should do?

Cheers Andy