The problem is trying to isolate Engine Noise while it is running... from the very temporary sounds created from the moment that the Starter Solenoid fires the Small Gear into engagement with the Fire Ring Gear...and it oscillates because the Ring Gear is Broken at its Center at the Crankshaft and thus ..it tries to get "Out of Round" as it spins the OUTER Flex Plate. Meanwhile... the Center of the Flex-Plate-Crankshaft is doing NOTHING... as the the outer portion passes over it and struggles to lock it up... like "Sharks Teeth" trying to bite into the Broken Flange of the Flex-Plate and begin to turn over the Motor.
Sometimes ...those 'pieces' will wedge themselves tight enough for a while and NOT make the same Grinding Noises... a Very Deceptive and Sneaky way to conceal the problem. If it "Locks Up" you will NOT hear any problems of this weird noise... because the Engine RPM and Torque will manage to power-fuse them together when under acceleration.
But just know ....that when this problem finally gets bad enough to break loose again and give way... You'll hear the Starter turning rapidly ....but with no "Shark's Teeth" left at the Center sufficient to Grab and Hold the Crankshaft... soon enough ...you'll notice that the Engine will NOT Turn over and Start.
Please be aware... that the OTHER phenomena is that if this "Loss of Teeth"
Securing the Flex-Plate -->to the Torque Converter--> to the Crankshaft occurs WHILE YOU ARE DRIVING ALONG... then you'll notice that the Engine suddenly Races under accelerating RPM as you step on the Gas Pedal ...but the Vehicle does not respond in kind and will soon STOP MOVING WITHOUT DRIVELINE POWER... a feeling akin to having the Transmission virtually... Disappear from under the Vehicle. And.... as soon as you coast along to the nearest curb... that will be as far as you can drive ...without the help of a Tow Truck. There is no telling when this problem can rear its Ugly Head.
There is no reason to rush to perform any other Videography this late on what must be a Very Frigid Night for you, Mike. But I will say, Congratulations to You for taking the time to do this. I suspect that many other Members will very much want to View, to Listen and to Comment about your First Video for other clues they might see and explain any other possible problems they might discover.
When the 4.2L Engine in the Trailblazer-Envoys is Idling... with some variation to your own circumstances ... a Confirmed Cracked Flex-Plate will sound pretty like much like this one does ...and please notice that the VOP (Video Original Original Poster) has removed that aforementioned "Round View Plate" allowing even more of the "crackling and tapping" noises to be heard: