I had a similiar thing recently on my 182, I WD40'd the bottom ball joints and that seemed to get rid of the noise (although I'm told it's a sealed unit, so by rights it shouldn't have). A guy at work said he had the same thing and it turned out to be a fractured suspension spring. Then a couple of weeks ago one of my springs snapped, looked like it had been fractured for quite a while (rust in the split)... so I guess it could have been either :S