I just fitted some 20" vossen cv3's and coilovers and rear adjustable tie rods. lowered a few inches from standard height. i'm getting a banging noise in the rear when i go over a cats eye on the motorway, its only when i hit them in a certain way. its not the wheels rubbing the arch its definitaely a metal clunk or something. anyone any ideas? car is a 07 cayman

 

check the muffler inlet pipes.

 

As Joey said I would check the muffler inlet pipes but also make sure that something isn't installed incorrectly or needs to be tightened.