IIANM... This is the Video suggested by
@Mooseman ... and I Strongly Caution you from either standing near or laying under the vehicle while the Engine is Idling ... because without the Three Torque Converter Bolts in place to restrain the Flex-Plate... if it lets go and flies off of the Tail of the Crankshaft ...you might be very badly injured. Please use the same technique you used in your first video to videotape another Walk-Around while allowing the Engine to run at a Low Idle so we can all hear and see what you are experiencing. This fellow was taking his life in his own hands when making this video ...and he may never know just how lucky he really was:
Please don't despair about this First Stage of a very necessary "Exploratory Surgery" for having any lack of obvious cracks in the "kidney bean" shaped holes of the Flex-Plate. There may yet be some cracks hidden in it from view under the (8) Hole Thrust Washer that will be revealed when you remove the Bolts, Thrust Washer and Flex-Plate. However... I am in full agreement with
@Mooseman that before removing the Flex-Plate... Starting the Engine to a LOW IDLE ONLY as this engine can be damaged if you accelerate the Internal Rotating Assembly when it is UNLOADED.
By this I mean... Normally...the Mass and Weight of the Torque Converter bolted to the perimeter of the Flex-Plate prevents this condition from happening under ordinary operating circumstances. When you first start the engine at Low Idle... should you hear a Ringing Sound ... like a rhythmic Brass Bell noise... then the Prime Suspect still remains The Flex-Plate. Its important to realize that there are other dynamic forces that get applied to the Hardened Steel, Hobbed Ring Gear Teeth creating enormous stress to rotate under the influence of the Starter Motor that can affect the "Trueness" of the Flex-Plate.
If you examine the edges of the Ring Gear teeth and they appear ground off or "Chewed Up"... this would be serious evidence of an alignment problem between the Starter Small Gear and the Ring Gear during hundreds or possibly thousands of Engine Starts... and once again... if the Flex-Plate is warped...it will remain The #1 Suspect.
While the three Torque Converter Mounting Bolts positioned 120 Degrees apart can resist some of this stress... if the Torque Converter is worn about the internal shaft fitting into the Transmission... the forces would sufficient over time to cause the Flex-Plate to Warp out of its normal orbit. So if you hear that "Bell Ringing" sound... and then replace the Flex-Plate afterwards and then re-start the Engine at Low Idle and while listening for any repeat of those same, rhythmic and you hear only silence... then you will have solved the problem.
If however prior to the replacement of the Old Flex-Plate, you CANNOT hear any unusual Knocking or Ringing sounds... then the Onus shifts from the Flex-Plate and Engine Internals... over to The Torque Converter... which is capable of "swelling" or Ballooning up under heavy fluid pressure caused by the Transmission Fluid Pump under Centrifugal Motion moving the Fluid in between the vanes inside with the bulging effect of forcing the Flex-Plate to bend in and out very eccentrically.
If this turns out to be the case... replacing the Torque Converter is in order... right along with also putting in a Brand New Flex-Plate. And again, no despair should follow on as nothing can be investigated and repaired back there without enduring the difficulties of removing the Transmission first; regardless of exactly which component ultimately gets convicted along the way.
The important thing to do next... now that the Transmission has been removed ... is to ensure that any knocking sounds do NOT emanate from within the engine itself AFTER a Brand New Flex-Plate is installed as those would be sounds that can only be made internally when caused by a Rod-Knock and/or Spun Crankshaft or Connecting Rod Bearings. So what follows on from here requires just a bit of a Plan and some Organized, Investigative thinking about what to look for...and to do these actions... just one step at a time. So Take a Step Back... And Take a Deep Breath and know that everyone here will stay the course with you as you proceed.