rear noise

What is your question?

when I sit in the car or someone is sitting in the back seat the car is making a squicky noise. I have tried to see if this noise happends while I drive or go over bumps but I have not noticed anything. I took the car to the dealer and they told me that I need to change the rear heim joints at almost 600.00 dollars. I think they're trying to rip me off. what could this noise be coming from? It sound like the spring on an old mattress while jumping on it!....