There is something definitely loose. I would check all of your bolts again, INCLUDING the 4 upper bolts that hold the coilover to the body..
Also, check and see if your adjuster for the coilover is hitting against the car. My right adjuster flops around and actually hits on something occasionally.
One other thing to check, if you haven't relocated your E-brake cable, it might be hitting on your rim now that you lowered the car. Mine hit on both wheels when I installed the 1LE/ZL1 wheels.
The more I think about it, the more I think it is going to be your emergency brake lline hitting against the wheels and the wheel weights, or the adjuster, but man that sound sure sounds like metal to metal clunking like something is loose.