Another Clutch Question

End Play is important and should be checked...........but that event or initial movement of the crankshaft under first loading only happens once as the clearance is taken up........in extreme cases while shifting between gears you can get it to repeat.........................if it still thumps under constant load , it's probably the main bearings.....................Put it up against a tree or immovable object.

 

I'm sure it's not possible to do it from the back. It's a two piece seal that you push into a groove in the block and rear main cap. No access from the flywheel side.

 

Hey , at this point were all guessing.............the end play clearance sounds as if it is excessive , although your measuring may not be exact as you stated.....the noise changing sometimes happens when parts are loaded and then un-loaded.................unfortunately it may be time to pull the pan and take a look.......again it sounds like it's in the rotating assembly.........which can be crankshaft & rods and bearings and perhaps some loose pistons or skirts...
Tough call...........Hows the motor & tranny mounts?

 

The Buick V6 motors are well known for oiling issues...........along with that main and rod bearings seem to suffer. Other than a cursory look from the bottom it's pretty hard to really get into replacing bearings or seals from underneath...........not saying it can't be done it's just really hard.

Pull the Plugs..................Look at the oil when you drain it..........pull the pan and check for metal in the oil and or pieces that could be a piston skirt.........rotate the motor a little at a time while you take a small bar or screw driver and lightly pry the parts and look for excessive clearance while you look at the condition of each piston skirt............shake the rods and anything else you can grab, check the thrust bearing end play once again.........you can pull a main bearing cap off also..........but you should first do reverse torquing on all the parts just to see if any are now loose...........meaning if the part should be torqued at 100 lbs start the torque checking at say 85 lbs and graduate towards 100 lbs.
Then take a cap off.........I would start in the rear and take a hard look at the bearing.

 

this.

 

My trusty old v6 from my '70 ultimately changed hands in a '65 CJ5 after being beat to death in my Willys pickup....

Anyway, it had gradually developed a loud knock, and I didn't have the time or motivation to dig into it. I sold the jeep to a friend for cheap. I called him a week later and he found one bad rod bearing by doing the "grab and shake" technique. He changed that one bearing out and got another 10 years out of the engine before selling the jeep.