the harmonic balancer is the pulley that bolts to the crank. there is an inner pulley, a layer of rubber and then an outer pulley the belt rides on. if the rubber starts to separate, on start up the pulleys will slip on the rubber because of the drag trying to get all the other systems driven by the belt up to speed. usually 3-4 seconds. with the engine warm, the rubber most likely expands and doesnt allow for as much slipping, so you dont get a noise.
right now its not going to hurt anything, but be careful. once it lets go, the harmonic balancer wil shift, the belt will come off and you will lose all charging power and the power steering. it can happen at any time as well.
other option is a bad bearing, but it would then squeak constantly.
to test the harmonic ballancer, take a sharpie and draw a line from the outer pulley, across the rubber to the inner. then start it on cold, wait for the squeak to stop then shut off the motor.
if the lines arent lined up anymore, thats the problem. if they are, its somewhere else.
if elsewhere, take off the belt and check any of the pulleys for free spinning. all should spin with little effort.
06 Ranger XLT
5.5 Mid travel w/ 2.0 Fox Resi, Deaver F31 w/ 2.0's
8.8, 4.56's w/ Lock Right and 33's
McNeil Fenders and Bedsides