Is the belt not running smoothly or do you hear something because the belt is off center?
Lubricate
It is recommended to lubricate the surface under the running belt every 1 to 2 months or after every 50 hours of use with the lubricant provided. Check this in your manual.
Center
During the first few weeks of using the treadmill, you may need to re-center it. This may happen, for example, if you take off more with your left foot than your right. You should also center the belt after lubricating the tread. Check this in your manual.