When you change to coilovers, or lowering springs, you should... loosen the bolts that locate the rear beam to the body! the rubber bushes are winding up, as the car gets lower! Loosening the bolts will allow the rubber bush to re set itself, then re tighten, this must be done with the wheels on the ground, a struggle to do on the ground, but a doddle on a 4 poster ramp... whether it squeaks or not, this should be done, before it rips the rubber bush to bits!
Thom