The GMC CCKW

If your CCKW is up to snuff, all gearboxes in spec, differentials tight, smooth turning, no bent wheels, all tires matching, correct inflation, and you still have that mystery "whump, whump, whump" noise at speeds between 25 and 45 MPH, you may want to check the splines on your propeller shafts for proper indexing and wear, and that the tubes are not bent or dented. Also, check that the U-Joints for wear, and grooves where the needles contact the cross. If all is well, it may be a good idea to balance each driveshaft. I have had all of mine balanced, it got rid of the mystery noise at highway speed, and made/ for one smooth rolling CCKW.

BTW, it's a fact, Split axles run smoother and quieter than Banjos.
Joel

I must admit that I have so many noises that I don't quite what is coming from where

Thanks Joel. My truck is such a hodge podge of parts it makes all kinds of noises going down the road.