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Everytime I give it gas there is a metal against metal clunking noise underneath the cab. The same noise happens everytime I shift. What could it be?

U-joints?

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"Fork: an instrument used chiefly for putting dead animals into the mouth." - Ambrose Bierce

are you pressing the clutch all the way in?

is your clutch adjusted correctly? does it fully engage and disengage? (easy way to tell is if its hard to get in to or out of certain gears except first or reverse if your moving forward)

other things are the fly wheel bolts loose does it make this noise when you start the truck?

could be a loose flywheel bolt?

does it make this noise when you let off the gas at 4k rpm?

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Friends don't let friends drive stock...
76 mikado, emissions delete, dual alternator, rejetted 1.9 carb swap, non factory body lines
soon non points distributor, single (semi sealed beam) head light conversion

Pressing the gas, do you get one "clunk" or repeated "clunking"?
One clunk might be a motor mount. Repeated clunking could be (as DozerMek suggested) U-joints. If you have a long bed, it could also be a center bearing.

Jack

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'83 Isuzu P'up Diesel LS - Original Owner
JoeIsuzu@IsuzuPup.com
IsuzuPup.com

I would say---rear transmission mount, engine mounts, and U joints--in that order.

Well I found the problem. It was the carrier bearing and a u-joint.