I called the guys at Marlin and they told me in so many words not to worry about it. The sound is the bearing spinning, which is a sealed unit. And since it is performing normally it should not be a problem for as long as the clutch will last.
And the grease is for the movement of the bearing on the imput. So grease may not even get rid of that noise. So I'll leave it be for now.
Thanks for all the replies.
