The idler pulley and tensioner can rattle, the bearings do wear out, it's a fairly simple job to replace both mind and the parts can be had relatively cheaply (think I paid 78 quid for the idler pulley, tensioner and a new Gates belt). The only other source of a rattle that is typical with these engines is the vvt actuator which is attached to the inlet camshaft, but that tends to rattle most prominently on startup, rather than under load.
Get a good independent garage to have a look for you, checking for play in the pulley and tensioner is pretty straightforward.