I saw it- it was the radio. I guess it still is big, because in the Lance Chronicles, they were discussing the problem on where to place the radio on his back so that it wouldn't increase the drag while he was racing. This is when he was in Seattle testing out his suit in the wind tunnels. There was a section where they asked if there was any way he'd consider putting the radio in front of him rather than on his back. I thought he was so definitive about it- he was kind of slumped back in his seat, and he responded "nope".
That was definitely his radio.