A wierd suspension noise

I have an 06 Mazda3 in which I have updated the suspension with Eibach ProKit springs, the struts/shocks are still stock. At low speeds I get a creaking sound coming from the back over imperfections in the road, the sound doesn’t seem to be there at higher speeds. The sound also seems to be more prominent when the tank is empty/car is lighter. I used to have the noise when I had a coilover system on all four ends.

There just isnt much stuff to the rear suspension on most cars. Could it be dried out rubber bushings with some dirt in them, this used to cause the sway brace on my old Datsun to squeek a little. that thing had leaf springs that sqeeked too. It was possible I was told to get thin sheets of plastic and interleave them between the springs to stop the squeeking but I never did bother as that truck was a beater anyway. I guess my answer is I dont know, but thanks for the question as it makes me reminisce.

That makes sense, I was thinking it was something of the sort was likely. THX