The late model 235 has insert-type rod bearings that cannot have the clearance adjusted by changing shims like the older Babbit bearing rods could. By the time a knock becomes audible, there's a possibility that too much crankshaft damage has been done to correct the problem even by changing the bearings. Once the pan is off, inspect all the rod bearings for wear, and measure the crankshaft journals with a micrometer for wear. They should be round and straight, with little to no visible scoring, out of round, or taper wear.
Other possible sources of a knocking noise can be worn main bearings, cracked or loose-fitting pistons, wrist pin wear, or foreign matter or carbon buildup on top of a piston causing contact with the cylinder head. Does the noise only happen with the truck in motion? If so, it's probably not an internal engine problem.
Jerry
