rattle or ping when let off gas

Hi all i have a 98 sport that has a 93 wrangler 4.0 in it that had a claimed 60,000 on it when i installed it feb 2012. in the past month or so its been making a loud rattle or ping when you let off the gas after running up to speed.

First thing, do a spark plug reading and post up the pix

Can you determine if it is external to the motor or an internal motor noise? Have had several with exhaust rattle on throttle release and have seen those with bad motor and tranny mounts make noise on let off