SNR of Pa900, blow noise very low in background but annoying

[GianlucaG75]

Hello I own a pa900, what I expected from this class of product was a SNR very low , such as the best dac computer interface. But when switching on the keyboard after the factory data has been loaded and it starts the loading of operating system, in my headphone I can listen a very low underground blow like some electronic part has been connected to the output routing and is adding some noise. What I was expected from pa900 was a keyboard that when powered on were completely silent as if powered off. This blow disturb me when playing in late night or early morning in headphone with piano sound. Note: The mic is off.
Anyone could tell me if it is a design limitation or maybe my unit is faulty? If is a design problem, which Korg pa has a far superior snr??

Thank you