Do you know what pulley it is? Power steering, a/c, alternator, idler, tensioner...? If WD-40 fixes it momentarily I would say the bearing for that pulley is going bad. Try checking the the torque of the bolt to the puley and moving the pulley and checking for a lot of freeplay in it. Careful with spraying WD-40 on your pulleys, especially your alternator and a/c pulley. WD-40 will ruin an alternator and could cause the clutches of the a/c compressor to slip. Can also be bad for the belts and cause them to slip.