1. Check to see if there is a gap between the hinges on the lift back and the main frame which allows movement as the patient's weight shifts. The arrows point to this location under the lift back.
You will need to gain access to the plastic enclosure to observe the next locations.
Unscrew the two black fasteners that join the plastic at the foot end.
The enclosure is secured to the top frame at the head end and sides where indicated by the arrows.
2. There is a 1/2 inch diameter pin that rides up and down between two white plastic blocks. See the "2" location below. The pin should be virtually contacting both blocks at the same time so that it cannot shift and impact one side and then the other as the weight shifts.
3. Verify that the bearings ("3") retaining the center shaft are not moving as the weight shifts.