possibly it's vibrations from hard drive being amplified by the shelf you are using.
if you hold the tivo an inch above the shelf does the humming go away ?
If so, you can buy stick on "silent feet" (acoustifeet) from quiet PC or you can make do with bits of foam to break the resonant ink between box and shelf. with foam try not to cover any bottom air vents
As cybmole as mentioned it may be down to the surface the TiVo® box is sitting on. It will make noises due to the hard drive inside the box but it shouldn't be loud. Please let me know how you get on anyway.