A kerthumping noise with every rev.When rotated manually by hand

grinds? A;T BOX GRINDS IS NEVER EVER GOOD.
PARK GRINDS ONLY IF ROLLING CAR DOING SO AND IS AN ILLEAGALL ACT THAT.

WILD GUESS A/T BOX SHIFT LINKAGE IS A WRECK
ALL SELECTIONS MUST WORK
THE SERVICE MANUAL TOLD YOU TO TEST THEM ALL
PRNDL
PARK CAR DOES NOT ROLL ,HAND PUSHED
NEUTURAL CAR ROLLS PUSHED BRAKES OFF
LOW CAUSE 1ST GEAR TO MOVE CAR
rEV MAKES CAR GO REVERSE
DRIVE STARTS IN 1ST THEN 2,3,4, ETC.
ALL MUST WORK IF NOT, ALL, THE LINKAGE IS A WRECK.

Does it make the sound when you turn the drum by hand? It could well be the drum bearings that have gone.

check mud flaps on wheel arches in case bent and rubbing on tyres

check the tail shaft for failed "u" joints

If grinding noise is heard coming from front side, lift up both the front wheels and rotate the wheels one by one to listen to the sound, if any. If a noise is heard, replace the front wheel bearings as a set.
If the front wheels do not produce any abnormal noise when rotated, lift down the front vehicle. Lock the front wheels with blocks, release the hand brake. lift up the rear wheels and try to listen to the abnormal noise. If the noise is heard, replace the differential bearings and seals. Top up the differential oil to the mark.

Reverse is not "synchronized" forward gears are, so it does make some noise, try shifting to 2nd before engaging reverse to slow gear rotation and limit gear clash or grind.

If it sounds like a rumble from back wheels it may be the rear end going out. Mine has been replaced twice with used rear end. You should be able to confirm it by looking at the rear tires both sides of tire for an oil leak or wet spot on tire or rim.

The best place for a DIY manual is www.ALLDATADIY.com their info is very accurate.

may be a foreign object trapped in between the inner and outer tub. Major Disassembly required to split the outer tub. May also be the rear bearing in which case back half will have to be replaced.