My guess is one (or more) of the pulleys is out of alignment with the others. I worked for Buick back in the eighties and this was the case for alot of the Buick 3.0 and 3.8 liter V6s with serpentine belts. Buick came up with a fix months later where you would remove the harmonic balancer/pulley and install a shim (or shims) behind it (on the crankshaft) in order to bring the harmonic balancer/pulley in alignment with the other pulleys. It actually work! I must have done a thousand of them!