The opener to check the belt drive (Again, on some older Sears openers the belt drive on the motor was nothing but a large o'ring)
Pull the door disconnect and see if the opener runs smoothly without the door attached. If it does, then the problem is not with the drive.
If it is still noisy, the motor drive can be seen inside the unit after the cover is removed.
Make sure to disconnect the power from the unit prior to working on it.