When it rattles, listen.
Took wife to lunch today in Bonneville, as we left the restaurant we heard a faint tapping (I thought it was the blower fan). Got worse quickly and when we stopped for gas & opend the door it was loud but seemed to be only at idle.
Went home immediately and found large pieces of rubber missing from the balancer (found two that were over two inches by a half inch wide). Fortunately had a spare in the garage for a temorary fix until I get a new one.
Imagine it had been going for a while but the real failure came on quick. Since this may be more age and conditions related than milage (think this one just had about 85,000 miles on it), it may be a good thing to check for cracks.