So I've had this cluncking going on in the rear for a long time. Couple years. Finally figured it out it was my strut when my dad was bouncing the car so I could look around. I replaced the stock struts about 2-3 months ago. These are KYB GR-2's with WBS springs on them. Band new mounts and everything. I built them myself but I believe I did them right and tighted everything down fine. Also used pogo washers. Thing is the stock struts were making the same exact noise on the same side. I'm not sure what would cause both to make the same cluncking noise. Took a video of outside the car and inside the trunk. The inside isn't bouncing either so the washers are doing their job.
*edit* deleted videos from youtube since the problems been fixed.