Cool concept, but I'd lean toward upping the cost and the p/t, so that the two abilities have better synergy together. For the trample to matter, it has to be blocked by a creature with <3 toughness, but anyone who would block the Muckraker with that low of a toughness, probably is doing so because he can kill the Muckraker in the process, therefore nullifying its untapping ability. By making the Muckraker bigger, you give an incentive to block with a low-toughness creature (prevent some damage, while still allowing the ability activate. As a side benefit, a larger defender is more relevant anyhow.

P/T should go up, at that CC. The double green means it's hard to splash and the low toughness means it dies too easily. Why would you pick this card, if it's only purpose on turn 5 is to die? Compared to Game-Trail Changeling or Fangren Hunter, this pup is a smaller dog than it should be.

Before anyone starts griping about the morph cost, two notes:

Green is getting "medium strength" morphs in this set. Making a 3/4 trampler as a combat trick is pretty solid, moreso as a common, so the cost gets set where you have to think about it.

Second, with Way Stations and the (((Pendarvian Scout))), there are still ways to flip this guy up on turn 3. Not something I want to see happen often.

No, this makes sense: the scaling cost of the morph means you get a better creature sooner than you would on turn 5. As a common, this is works. It isn't good, but it's at least giving players a reason to put it down as a morph creature.