Could be any number of things, without seeing or hearing it could very well just be a mount.

If you still have the bilsteins, they are notorious on the GTB's for knocking and banging, talk to Stuart could be one of them starting up!

With your wanting a general check up etc etc from your other post, I would say get it looked at sooner than later, if it turns out to be something nasty you may catch it in time.

Good luck with it, hope its nothing drastic.