Could be caused by vibration from a cooling fan running at high speed, or the hard drive, or the mains transformer. Try a software refresh to rule out a glitch causing the fan to run at full speed.

To do this, first switch the box off at the plug and wait for 5 minutes. Then press and hold the backup button on the box while switching on again at the plug. Keep the backup button pressed until all 4 LEDs light up on the front panel (can take up to 60 seconds) then follow the on screen instructions.

If that doesn't fix it, I think you'll need to call Sky and arrange an engineer visit.