My treadmill belt keeps pulling to the left?

Ok i bought I used treadmill from a family member and I see the belt had pulled way to the left..Welli got home ajusted and tighten it up and ran on it for about 30 minutes and right back to the left it was again..I centered it a couple times and after about 30 minutes it pulls back to the left.I removed the cover over the motor and everything looks pretty good besides some belt dust but not a lot..Now there is a screeching sound that sounds like its coming from the top roller and the roller looks like it is kinda wobbly a bit when you close one eye and look real good at the edge of the roller..Also you kinda feel a little viberation sometimes like maybe a bad burn or someting..Any ideal what may be my problem ?? The name on the machine says Pro-form 485 PI

Albert
January 2009