My car is lowered using Ford C springs. I installed them last December and it started making a terrible noise from the rear about 4 months ago. I had taken everything out of the trunk and it still made the noise. It sounded like it was coming from the right rear, so I checked in that area, everything was tight, but it still made the noise. I looked around on here and found all sorts of suggestions, so I tried them all. I pulled the rear seat, all of the panels out of the trunk, and the spare tire and jack. I gutted the car and rode around. Still had the blasted clunk! Gutted though, it sounded like it came from the left. While riding around, I noticed that road noise is terrible and my SLP LoudMouth sounds super aggressive, hehe.
So I went home and jacked the car up and tackled the left rear. I finally found that the bolt to the swat bar that is closest to front was not even fingertight, it was just hanging there. That was my last option, so I tightened it and rode around. CLUNK GONE! I could have saved gutting my car and wasting about 45 minutes if I had just checked it in the first place.... but I'm not putting the interior back in until after I go to the track next weekend
So I went home and jacked the car up and tackled the left rear. I finally found that the bolt to the swat bar that is closest to front was not even fingertight, it was just hanging there. That was my last option, so I tightened it and rode around. CLUNK GONE! I could have saved gutting my car and wasting about 45 minutes if I had just checked it in the first place.... but I'm not putting the interior back in until after I go to the track next weekend
.Did what you did by removing things,but did not help.So i guess i will do what you have stated...check the sway bar.Another noise i get is from the Mach 460 system(speaker,and amp inclosed case in trunk),i always have to retighten this freakin' thing.Time for some lock tight!!!! 
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The mount is positively stuck to the trunk, no way you can move that thing without some serious effort.