Just ended up replacing my rear wheel dampers after changing the clutch and having the rear off my X. Couldn't figure out where this noise was coming from. Sounded like the rear of the engine. By accident I noticed the dampers were shot as I was putting the rear wheel back on. It must let the gear get out of square because it sure mad some noise. The noise traveled up the driveline which through me off. Got an email from Duc who had the very same problem. I had 47K on mine and Duc puts on more miles than me so would look for it as the miles get up there.