That's a good suggestion about going to Auto Zone, or any major parts store. The inner and outer parts the harm bal are bonded by rubber visible on the most forward surface of the pulley assembly. You do not have to see into the recess where the bolt is nor do you really see the rubber as it is thin. A white line accross this area will be very obvious. I really doubt that this where the problem is though but it is nice to know when and if it slips. The white line will tell you at a glance, better than the audio box method.
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David Hunter