370Z Annoying Rear Clunk

FrankEtier

Hi Guys! My '14 370 with 49K on the clock started to develop an annoying low speed, under 30mph "clunk" coming from the rear of the car. Mainly noticed when driving on concrete roads with spaced expansion joints. This started after I installed Eibach lowering springs and still remained after switching to KW V1 Coilovers. The only other rear suspension mods are SPL camber arms and Z1 Subframe collars.
Yesterday I did the following trying to determine the source of the noise:
- Removed everything from the rear of the car. Jack, Spare Tire, Bose Sub, Styrofoam fillers, etc. and drove around neighborhood. Low Speed Clunk remained.
-While driving around neighborhood I opened the rear hatch just to see if the latch mechanism could be the source of the clunk. No luck, driving low speed with rear hatch opened the clunk remained.
-Went back home and put car on jack stands and checked every rear suspension bolt for proper torque. Found nothing loose or anything that appeared worn. Drove car again and low speed clunk remained.

I've determined the clunk could be something behind the rear bumper but not on the interior of the car. Or this could just be a noise I'm going to have to live with, price of a lowered car with subframe collars and performance parts and 30 series rear tires.

Has anyone experienced anything similar to this? If so what did it turn out to be? Any ideas of something else to check would be appreciated or just telling me to suck it up and live with it would be welcome also.

Thanks

__________________
‘14 Topgunz Speedworks Rotrex Supercharged VR30 7AT 370Z Magma Red.
@liquid.magma