A whole host of things it could be. It could be bushes but I would think if they are knackard they would 'clonk' rather than squeak. If they are poly bushes they could squeak anyway. Check the springs and the seating of the springs top and bottom. If you have progressive rate springs the coils can be close together. My Eibach springs have a plastic protective tube around the lower part of the spring to stop they touching metal to metal.
That should get you started.
