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.

Lack of lubrication

Lubricate as required. Refer to Spring Maintenance .

The shock absorber is faulty.

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