I had the same problem and it turned out to be small stones that had settled in the tray that the suspension springs sit on. We assumed we picked them up from a gravel car park. All rods and joints and shocks were in perfect order and the garage could only find the stones. Once removed the clunks disappeared. We now have the clunk back again on the passenger side front after months of clunk free driving. If it is stones again at least our local garage will know what to look for now. It is worrying and the noise is very loud, so look for small stones in the suspension tray first, you just never know!
Geoff Denny, April 2013