My '77 GMC K2500 had been lifted,wheeled,and beat before I bought it--I took the cobbed up front springs that were not even for a GM out of it,ditched the lift blocks in the rear,the 36" Dick Cepeck "egged out" tires,and put stock 3 leaf front springs back in it,steel 16" rims--basically returned it to stock..
That truck had many sheared rivets on the front crossmembers,I put grade 8 bolts and locking nuts in their place,plus welded them where possible....after that the truck rode and steered nicer--but it always had a disturbing Tha-Kunk kind of popping noise coming from the front,most often it did it when you turned a corner,or the steering wheel sitting still..--at least I thought it was up front--you could also "feel"it in your feet when it did it..
First thing I feared was the steering box had ripped the frame--but nope,I unbolted it--no cracks or hogged out bolt holes,much to my surprise--I expected the worst,seeing how the truck was "rigged"..
I checked all the spring bushings,center bolts,u-bolts,even replaced all 4 ball joints because the lowers had just a tad of detectable play..if anything the noise worsened !..all the body mounts had been re-done and someone put 3" aluminum solid blocks in for a body lift--I removed the cab bolts,and put rubber between the blocks and mounts--no improvement...
It was not until several years passed,that one day while lying under it draining the oil,waiting for it to drain,did I notice a bolt on the transmission crossmember was not tight,I saw daylight between the frame and lock washer on the bolt--went to tighten it--it snapped right off,it was whittled down to nothing--it was then I realized someone had not only used 3/8" bolts instead of the stock 7/16" ones,they also removed the part of the crossmember that goes to the upper lip of the frame rail,which is normally riveted on by the factory..and never put it back...
I ended up having to drill out the holes in both the frame and crossmember that were all hogged out,to 1/2,and after I installed new bolts,the noise vanished,and maybe it was just me,but the truck also "felt" firmer riding,and steering felt tighter,like the frame had been flexing in that area all that time..
I never did bother to find or install the missing upper part of the crossmember for the transmission..
That was the last place I suspected the noise originated from--if anything the left front shock absorber and where it bolts to the frame was the next two things I was going to inspect,that is where it sounded like it came from..I've seen more than one frame crack at the upper front shock mounts..
