Squeaks in the Suspension

I am about to buy a 2002 318ia. It has done 44000km. The car is inn tip top condition apart from 1 glaring issue:

When I rock the car from the trunk, both rear suspensions have a very obvious squeak. much like the "shaggin wagon" squeak on both the down motion and the upwards/return motion.

I am trying to diagnose the problem and work out the replacement cost if it is a problem and if this means there are some other skeletons in this car's closet.

The squeaks are not audible when the car goes over bumps though. Only when I actually get out and push up and down on the trunk area.

Any one have any ideas/similr experiences?

Thanks