2002 AUTO 4.6 loud banging/grinding

Hi, i’ll Explain a bit and offer a few suggestions. The Shop hooked up and ran a full Diagnostic Test on the Trans, or just looked, listened, prodded a little, and ate Doughnuts & Drank Coffee, then offered their Opinion?
Going somewhere and getting a solvent flush cannot get to the debris that is trapped in the valve body; in fact, it may dislodge it and move it to a worse place... Getting a fluid flush if the unit is operating normally is OK, providing you include a filter change as well.
Did the Motor get hit when it went under the Truck? The Balancer May be cracked, or damaged- causing vibration issues.
Is there any CEL’s? A bad TCC Solenoid will generally throw a DTC. P0740 through P0744 are the Codes related to that, I find it a long shot as the TCC Solenoid is responsible for supplying fluid to the TCC, for Torque Converter “Lockup”, so your Car feels engaged solidly in each Gear. Failure generally causes Shudder- no Clunks or “Bangs”.
The torque converter simply transmits engine power to the Trans. To prevent slipping at speeds exceeding about 35 mph, the TCC ‘locks’* the engine's output drive to the transmission's input shaft to achieve a 1:1 ratio - meaning 100% of the engine's power is now reaching the Tranny. Driving 30-45 MPH would be the most erratic area if the TCC or TCC Solenoid is bad.
Driving a vehicle with one that is damaged can be risky as it can literally disintegrate when it breaks down – it’s not there, yet- but can take Months of daily Driving to completely fail, adding metal debris into the Trans. fluid. The contaminated fluid can then make its way into the Tranny and cause significant damage or even complete failure, turning what could have been a 200$ converter R&R into an expensive Trans./possible Engine repair.. If it’s bad, it can make metallic noises.
Are you certain it’s not a Backfire back through the Motor, or Exhaust leak, and the Trans. is just Shifting strange
Unclear if this was as result of the Accident, or a previous issue, or afterwards(?) That’ll help..
You may have (2) issues, the Torque Converter damage (if truly damaged- it’s likely out of balance, and should be replaced) may cause more Transmission (even Engine) Damage. by irregular or erratic Shifts, Flexplate May be cracked or bent- sometimes presents as an uneven, loud noise when Starter Motor rotates. A cracked Flexplate May sound like a Bearing Knock, all the time, or when the Motors under a heavier load- like accelerating. May even be loose. Is there a Vibration felt all the time?
Start the Car and hold it at 1,000, 2,000, 3,000 RPM’s for 10 seconds each..feel a Vibration? Drive it in 1st Gear and try the same.
The shift solenoids (separate from the TCC Solenoid) regulate the flow of hydraulic fluid for shifting. A problem with one or more of the solenoids sticking or not cycling correctly may cause a lack of fluid pressure (vital for proper operations within an Auto Trans.) resulting in hard, soft or delayed shifts. It’s Shifting Normally, just noise- correct?
You ever run Synthetic Oil in the Transmission?
A ‘Clunk’ usually occurs if there’s Mechanical damage, i.e.. worn main shaft bearing, Shaft Spline wear, ID Gear Splines worn, etc.. However, if you’re lucky, the clunking sound may be caused by something like worn or damaged U-joint(s) on the Driveshaft.
If the accident bent the Unibody, the Rear end itself could have sustained internal Gear & Bearing Damage by Driveshaft thrust, or affected Pinion angle. Either could produce noise described.
Raise the Car up, or reach under- check for excess lash in the Rear/universals while grasping/rotating the Driveshaft back and fro slightly with the Parking Brake on.
With the Car Rear Jacked up & on Stands, Jack the Rear end up, carefully, to ride height. Check how far the Driveshaft is inserted into the Tranny (accident may have altered distance between Transmission and Rear) and put the Car in Neutral and release the Parking Brake, spin a Tire both directions and listen for unusual noise(s)& Clunks.
Look for loose or broken Transmission mounts & Bolts, broken Bellhousing Bolt(s) May cause this, check the (2) Fluid lines to Radiator/Trans Cooler from Transmission aren’t bent or kinked, crushed anywhere, limiting Transmission Fluid flow.
The answers to the questions will be very helpful, the Car is shifting Normally and Trans &
Engine Temperature stay normal, you have something out of balance. Please include how well it Shifts, anything unusual aside from the noise.
If the Unibody was bent. I’l be on everyday, and other Members may also comment. Good luck!

I find it ironic that the accident occurred, noise you have may be caused by 25 different reasons from that type of Collision.
Was the Frame bent, or was it unchecked for one reason or other? Was it Totalled? You checked the Tranny lines to the Radiatot, no obstructions inside?
The Valve Cover being struck, that damaged may indicate possible noise due to Valvetrain Damage, as seen; the Cam Timing Sprocket being positioned high above the Valve Cover & other components, a hard strike may bend the Gear & thrust the entire Cam backward and misalign Tensioners below the Lobes, and bend or break the Tensioners(s) themselves. Pic:

The Timing Chains are VERY long to begin with, doesn’t take much to upset the balance.
Did you check the Starter, examine the Flexplate housing that Bolts up looking for any interference with the lower back on Shroud(s) removed, or by a Buddy rotating the Motor by the Crank Pulley with a Socket & Ratchet by hand, (Coil, Battery disconnected)
On JACKSTANDS, not just a Jack.lift Car, did you try the suggestions I’d left you in the 1st email? What were the results? Universals,?Gears, Pinion Bearing & angle OK?
A few other Diagnostics and quick solutions, that’ll point you in the right direction. Torque Converter Test....Transmissions can shudder in a very similar manner to torque converters. To determine whether the shudder is tranny or TC related, run your vehicle for a little while till it begins to shudder and immediately park it. With your foot on the brake, put the transmission in Drive, and apply light throttle. If you feel shuddering, then it's likely the Torque converter. Most Transmission problems are not serious,!but will become serious if not repaired quickly.
Unless there is physical damage to the converter or transmission from the accident, “shudder” is a fairly easy problem to solve. Once the transmission fluid has been overheated enough times, it becomes permanently de-polymerized and need to be Cleaned, in some rare cases- replaced. It’s not difficult & You can do this yourself by draining the transmission and flushing it, several times. or taking it to any competent Shop for Transmission Maintenance.
The alternative repair is a Transmission Fluid Viscosity enhancer. Lucas brand is the most popular and works well, but usually the most expensive, contains friction modifiers that the stock Dextron does not, don’t use F type in your 02’ GT!nZuse the Additive & Non Synthetic Oil. Engine is hot and running when Oil level is checked? (Sorry, has to ask- some do not do this).
I’d also replace the Rear end Lube, Whale Oil worked the best- good luck finding that, use a high quality OEM Oil.Good luck! John

Glad the noise is isolated to the TC, and you are moving along again! Might want to Find a new Converter, plan, etc, stay ahead of it while time allows. Less headaches.
Ultimate would be a Trans from a Wreck (Good Shop Checks out over time, Costs little) so you have no snags- when need be.gPod luck!