what could be a possible cause for rattling noise under my car ?

I have the same issue with my 08 tc. For me it was a simple fixture but depends on when the sound occurs. For me when I accelerated, I heard the noise. This problem was fixed by replacing the exhaust harnesses (very similar to metallic hose clamps). Time and wear simply cause this piece to rust away and its main purpose is to keep the exhaust pipes from dancing around as you drive. When you accelerate, the pipes vibrate against the old and loose harness. This is just one possibility among water pump going bad to misfiring/old spark plugs. Process of elimination so try the crappy rusted harness theory first! Try looking for either pipe harness or a hose clamp online or at home depot. Simply replace the old with the new and the rattle should stop. Good luck!