At this point... I am dis-inclined to suggest that you open up any areas of the Engine-Drive Train for performing serious mechanical tear downs to locate this problem. Given your reticence to run headlong into tearing into the "Undiscovered Country" of these Potential Problem areas... the practical alternative would be to take it to a Reliable Auto Repair Shop and ask them to do a "Flat Rate" Investigation... BUT WITHOUT AUTHORIZING THEM TO DO ANY ACTUAL REPAIRS. You simply want them to Identify any problems... but absolutely take No Action of Component Dis-Assembly.
Once a Trained Mechanic has the Vehicle on the Lift and Identifies what he thinks the problem is... Then you can decide if it would be cheaper and easier in the long run to pay a Professional to do any repairs. Do NOT allow the Shop Mechanic to suggest doing ANY other Repairs while they hold your SUV "Hostage" on that Lift! Make sure they agree to a "Flat Rate" Investigatory Fee.
If you insist on going forward... I would suggest that you NOT "Start" with the "Starter". Instead... Safely elevate the vehicle and after ensuring that it rests good and solid on Jack Stands. Next, slide some Cardboard under the engine area for you to lay down on and move yourself into a position to "I-Ball" the mating areas underneath and between the Back of the Engine Block and the Bell Housing-Transmission. Now... look Dead Center at the Back edge of the Crankcase for a Small Round Black Plug ...and Pry and Pull it out. That Hole is the Access Port that allows you to slip a 15 MM Socket through there and when the (3) Torque Converter Bolts get aligned... You can check whether or not they are Loose.
But before you can do that....look underneath and a little further back and your will see two of the same sized Black Rubber Plugs connected along a Narrow Slot between the Back of the Engine Block and the Transmission Bell Housing. If you Pull that (those) Plugs out... you will be able to get a Large Flat Blade Screw Driver in that area...and Carefully Pry Rotate the Teeth of the Flywheel in that area... Turning the Flex-Plate Clockwise when facing the front of the SUV.
At the Same Time... Use a Bright Flashlight to observe in the small hole as the (3) Torque Converter Bolts come gradually around and into view in the port flange os the Crankcase-Oil-Pan. Slip your 15 MM Deep Socket over each TC Bolt and TEST to see if any are loose as you SLOWLY apply a Clockwise effort with a Ratchet in Very Small Increments to observe if any of them move at all.
If they do... Back the ones that are loose outwards a bit... and then USE A TORQUE WRENCH to Tighten them Down. It is VERY Important that you AVOID OVER-TIGHTENING THE TORQUE CONVERTER BOLTS.
@Mooseman 's Engine FAQ in the GM 4.2L Engine Section has a listing of ALL Engine Torque Specs for tightening fasteners properly.
After tightening any TC Bolts that are loose... REPLACE THOSE THREE RUBBER-PLASTIC PORT PLUGS and then Start the Vehicle at idle and allow the Engine to completely warm up B4 you re-do the 3,000 RPM "Squeal Test". If the Sound has abated... Problem Solved. If not... I would NOT go through the Aggravation of having to Pull the Starter on a Trailblazer-Envoy unless my plan was to just replace it with a Brand New HQ Starter.
There will be no way for you to diagnose the Strength or Weakness of the Bendix Gear-Spring Mechanism... and since BOTH the Small Gear AND the Fire Ring Teeth at the Outer edges of the Flex-Plate are made of Very Tough, Heat Treated, Hardened Steel... there may NOT be any signs of their Friction Contact doing any damage.
