Does Rumble Hotheaded's recycle come from the board or can it be from your hand?

You can only recycle a unit from the board (base or battlefield) to activate the ability of Rumble, Hotheaded.

According to RiftJudge FAQ #3053 and FAQ #9333, the term "unit" without further specification refers only to units on the board. Units in your hand are in a non-board zone and are not considered "units" for the purpose of this ability. Additionally, Rumble's ability specifically requires you to recycle a unit to play a Mech from your trash, not from your hand.