If the noise is coming from the walking belt this could be due to sand or grit getting under it. These belts are big electro-magnetic generators. The attract sand and grit to them. Usually it will make a high pitched sound when there is sand under the belt, especially if there is a mylar or plastic sheet under it. If this is the case the sheet can easily be replaced.

Thank you, you are correct. How do you replace mylar sheet on the treadmill? I have NordicTrack C2000 . cannot find info anywhere!