I just got back from the dealership, and at first, they told me the sound was too faint and that I was being too picky (!). After pressing them to look further, they ejected the Navigation disc, and that stopped the noise. Apparently, the noise was coming from the periodic scanning of the disc. But, you have to have the disc inserted in order for the Nav to work. They told me that was the way the system was designed, but I find it hard to believe that a functional system would be making that noise constantly.
How could Toyota see that as acceptable?
I told them that the problem only appeared 6 months after purchasing the truck and that they need to look into other 2010 4Runners at the dealership to see if they make the noise. I left the truck with them and am waiting to hear back about what they find in other vehicles. At one point, they suggested that I use the radio to cover up the noise, which I said wasn't a real solution.
I will keep you posted...