Bang on the exhaust with your hand to see if you can hear where it rattles. It may just be the internal baffles in the exhaust, or it may be metal shield around the catalytic covertor rattling, or it may be the internals of the catalytic convertor. As long as it's not blowing, & nothing is going to drop off & if it passes the emmisions part of the MOT, then i would put up with it.
jim
July 2010