Make sure belts are properly adjusted; loose or tight treadmill belts can cause the machine to malfunction. A misaligned belt can create squeaking, screeching and whining noises. Belts can get off track if your treadmill is not installed on a level surface. Follow your treadmill owner's manual regarding belt lubrication. Maintain a low level of belt friction for optimum performance and to protect your treadmill's motor

It is probably the join on the belt itself as it passes the over the roller. It could be a sign that the belt is breaking.