I recently had a rattle on my single pipe exhaust and it turned out to be a crack around the rear pipe where it exits the silencer. No noticeable change in exhaust noise, but a rattle every now and then. Had it welded and now cured.
I think they may be prone to cracking here, as the metal on rear of silencer is quite thin.
Worth a close look - it's not always that easy to spot.
Otherwise, you can probably pick up a second hand replacement quite cheaply with an ad in Wanted.