Just did a flexplate on my 2500 Suburban. You can usually investigate those by varying the load with the transmission. You can hear a change by shifting into neutral drive or reverse and see if the knock changes.
The 4L60e thru 4L70e have the 2 inch inspection plug, the 4L80E has a removable 6 bolt inspection cover.
The bulk of the damaged flexplates on LS drive trains crack around the 6 bolts that the plate attachs to the crank.