I have 2 XTM .21 Maxx's!
And that is a tuff one! Does it sound like metal to metal? If it does maybe it is the spur gear rubbing against the clutch bell? I had this problem with one of mine and that is what it was! It was too tight a fit and I had to cut out a bit of the chassis where the flywheel is so I could move the engine back just a tiny bit!