I think I know what your problem may be. I have a persistant clunk in my car that I have described before. It seems to be due to some of the trailing arm supports that are attached to the body (I can't seem to remember if it was the lower or upper ones that is the issue, you may be able to find the discussion in the archives or search). But to summerize, one set of the trailing arm mounts on the body is for some wierd reason made of thinner sheet metal than the rest of the car. This causes the holes to become worn in an oblong fashion letting the bolt shift about a millemeter or so when you make sudden starts or stops. I have been able to temporarily get rid of the noise by torqing the trailing arm bolt/nut to attempt to clamp the mount harder, but it always gets loose again and the clunk persists when I make a quick stop or start. The only solution would be recreating the hole in it's proper round form and reinforcing the metal somehow. There are a plenty of ways you could go about doing this, but I haven't bothered trying yet.