Have u try playing with the Digital Audio output setting of your STB? Check if you can select another option, mine got LPCM & BS option.
If possible, borrow another brand of STB from friend/relative and see if problem still persist. Lastly contact Hyundai for help.
R u using the same Hyundai model and dun encounter tis kind of sound issue? All kind of mentioned options tried.
Brought my unit to Hyundai and they changed one for me, came back and tested still the same .. no extra loan set can get .. waiting for my redeemed NMS set to come to see if the same noise still there