A quick noise test that I do is to spray some WD40 on the belt while engine is running. If it's the belt making the noise you will notice an abrupt change in the level of noise. If there is no obvious change then with the engine off spray some WD40 on the idler pulley bearings (mounting screws). Then start the engine and see if the noise changes. If it does, change all the pulleys and install a new belt. Keep the old belt in the trunk as a spare.
If the noise persists then it is probably coming from either the PS pump or the alternator. Depending on the mileage of your xfire you could try flushing out the PS fluid and installing new before replacing the pump. The alternator could have a dry shaft bearing which can be replaced by an alternator rebuilding shop. Hope you solve your problem.
Jack