Belt Slipping Forward on Treadmill?

I have a question about a Proform 525 Crosswalk treadmill that I currently use for walking. It is a used treadmill not brand new. If I try to take it to speeds of [3.5-4.0] for walking the belt will lock a little and slip forward not backward on the treadmill. We have tried adjusting the belt at the rear of the treadmill at one time. I'm not sure if it could be the belt drive, front pulleys or rollers. Could it be the front rollers too? It also makes a fairly noticable zing zing sound on the right side of the treadmill while I am walking.
It is a nice piece of equiptment and I'd be interested in any advise on repairs.