HOW CAN I FIX MY SQUEAK MERC 310D 1998?

MY VAN WON'T STOP SQUEAKING, THE NOISE IS COMING FROM THE FAN BELT AREA, THE WATER PUMP, POWER STEERING PUMP AND GUIDE PULLEY HAVE ALL JUST BEEN RENEWED. IT IS A NEW BELT AND WHEN YOU PUT A TOUCH OF OIL ON THE BELT IT STOPS SQUEAKING FOR 3 MINUTES BUT THEN RETURNS. PLEASE HELP ITS DRIVING ME BONKERS!!!