Looking for some help...

Just a day ago I noticed my 2006 Roadster is making a kind of rattling noise coming from under the car (like metal clunking against metal)
It only seems to be when i'm moving at low speeds but continues for a few minutes if i pull over and rev the engine.

I have checked all the heat shields under the car and all are attached solidly.
As far as i can check without getting it up on the ramp the exhaust also seems to be attached at all points and solid.

I know this might be a long shot but was wondering if any you guys had a clue what it might be.