Hello. My '86 Volvo 240 sedan is low/sags in rear. I thought maybe what I needed to do was replace the rear springs. However it has been suggested that it may not be springs but may be rear trailing arm bushings that need replacement. How can one know/test to see which needs replacing ? I have noticed that when I take off from a stop sign or red light that there is a noise/creaking sound from rear. Thanks in advance for advice & help with this.