GM Service Manual Online
For 1990-2009 cars only
  1. Check all the mountings for proper torque. A loose mounting will cause a noise.
  2. If all the mountings are intact, bounce the vehicle by quickly pushing down, then lifting up the corners of the bumpers. Use the same amount of effort on each test.
  3. Ensure that you hear only a hissing sound. Any other sound indicates a problem.