The spring has loose U-bolts.

Tighten to the specified torque. Refer to Fastener Tightening Specifications in Rear Suspension.

The spring has loose or worn eye bushings.

Replace the eye bushings. Refer to Spring Bushing Replacement in Rear Suspension.

The spring lacks lubrication.

Lubricate as required. Refer to Fluid and Lubricant Recommendations in Maintenance and Lubrication.

The shock absorber is faulty.

Replace the shock absorber. Refer to Shock Absorber Replacement in Rear Suspension.