I agree with the above. It is likely that your throwout bearing is on it's way out. I've got a 2000 frontier with the 5 speed and about 200k on the clutch and am now experiencing occasional noise when releasing the clutch. Mine is kind of a high speed vvvvvvvvvvvvvvv sound. like a playing card in the spokes of a wheel going 200mph. I'll be doing a full clutch replacement soon.