[der Geist]

One possible thing is that it could be your shock. The sound you are describing could expain it however it usually always sound like it is coming from the same side and does not move around. It is highly unlikely that both shocks went at the same time. Did you look to see if any of the fluid has leaked out of the shock assembly? It will usually be apparent on the inside fender or right on the shock itself. When a shock loses it's fluid it can make a sound exactly like what you saying but whatever it is it sounds like it is related directly to the suspension or something attached to it.