Why is my treadmill screeching and what is it?

My treadmill is squealing and screeching from the rear. It is a Nordic track c 2000. The front roller and motor sound good, no noise, but the rear of the treadmill, especially where the belt moves around the rear roller is making a loud consistent screeching noise. It gets even louder when using it. Took roller out and lubricated and greased but still didn't go away. Do you think it is the belt or the roller? Will try lubricating the belt. Does the plastic mat under belt ever cause problems?