knocking in 95 3.8

[mayville1]

I just put new heads on my 95 3.8 mustang. And its been about 300 miles running perfect....... Then yesturday i was drving and as it winds out to shift i could hear knocking........ So i drove it home and by the time i got home. ITS knocking hard at idle. and it like pings sometimes 2... I need any info on what this could be.. OR help 2 figure out whats wrong with it....