Does the rattle seem to come from the front of the car or the back? Also does it sound inside or outside the car?

u say it backfires aswell so u have an engine problem, are u sure the rattling isnt cumin from the engine? if so it cud be pinking (pre-ignition)

I'd be tempted to check the spark plugs whn looking into the backfire...then the HT leads, coil packs and so on.
As for your rattle...it could be something daft like an exhaust rubber has worn so the exhaust rattles.

The other thing that may cause backfiring at a guess would be the timing is out...might be worth just getting the timing checked and a new cambelt and tensioner if these haven't been changed for a while. I think 40k miles is the advised interval.