Take a piece of garden hose and put to your ear. Carefully position it near different pulleys (with engine running) and see if you can isolate which area the noise is coming from. If it's the bearing in the belt tensioner it can be replaced cheaply. Other candidates are, alternator, power steering, Air conditioning compressor, fan clutch which can be very expensive depending on which one it is. Wather pump or bad timing belt pulley are also possibilities.

You should also check to make sure that the fan or belt is rubbing against something.