Chevy & GMC Vans

2000 chevy astro van with strange vibration. Pretty vague question but it vibrates at idle so bad it will spin a coffee cup in the cup holder. At just over idle and driving its fine. Motor has tons of miles 548k. Just started doing this awhile ago. No check engine light and it passes emissions. Like i said very vague question as it could be anything. My question is would a slipped ring on harmonic balancer cause this? If so is there a way of checking if the ring has slipped before purchasing a new one?? Thanks for any help on a really dumb question.