When I recycle with Rumble, do I recycle a friendly unit that is already played or from my hand?

To activate the conquer ability of Rumble, Hotheaded, you must recycle a friendly unit from the board (a battlefield or your base).

According to RiftJudge FAQ #3053 and FAQ #9333, "unit" without further specification refers only to units currently on the board. Units in your hand are in a non-board zone and cannot be recycled to pay for this ability.