I have read about the frame resonance you mention and I believe they called it "Ladder Wracking" or "Ladder Twist"This has been discussed before (the resonance/vibration from the middle of a GMT400 frame) though I can't remember the proper name for it. My C2500 Suburban had it mild at around 60-65mph - but it's got much worse and now occurs at 70mph - a useful cruise speed.
What has changed is that I rotated the tyres (as per handbook for four - spare not included) and a change of tyre pressures from 45/50psi F/R to 40/45psi F/R. I'm pretty certain it is the frame resonance thing - it is at a frequency much higher than wheel related. There's a remote chance that I got some underseal on the propshaft but I don't think I did. The vibration now is quite intrusive and I'd like to be shot of it.
Which if either of the above two changes has most likely caused the changes (in speed that it occurs and severity?
If tyre rotation - will it right itself as the tyre bed into their new positions?
If tyre pressures - is front or rear the bigger contributor? (If the fronts then I have scope to raise the pressure there but not at the rear (psi limit is 44psi on sidewall)).
All advice welcome! TIA.
That might even have been from a GM TSB.
The frame on these trucks is pretty much built like a ladder and it can twist.
Have you had someone re-balance the tires?
Truck tires can be difficult to balance properly.
The correct procedure if you have a tire that is way out of balance is to unseat the tire, rotate it 90 degrees and try again.
A decent offroad shop will do this.
The average tire shop will just stick a bunch of wheel weights on it and call it good enough.
On alloy rims, they might have half the inside of the rim covered in tape weights and not really prep the surface.
Those weights get flung off one after another as you drive.
Is that vibration actually at a higher frequency or are you hearing the higher resonance tones of a set of vibrations that have now moved to the front of the truck?
Imagine if we built some monster sound system and attached it your rear axle in the back of the truck and behind your ears and played War Pigs untill the truck shook.
Then imagine if we took that same system and installed it under the front axle and played War pigs.
Same song through the same system but a very different experiance.
It would not surprise me at all for that percieved vibration to change when rotating tires.
I would assume that it was a tire problem or a rim problem way before I would worry about a ladder jacked frame.
I think you have a tire problem or a motor mount transmission mount problem.
Swap the fronts side to side temporarily and see if the vibration changes.
And obviously, make sure you arent chasing an out of round brake drum or a sticky caliper.
Swapping a poorly weighted rim onto an already sticking brake can change how that brake contacts the surface.
And, just in general I highly recommend to everyone that you spend the money to buy a good quality digital tire pressure gauge with a bleed valve feature on it.
The 3 dollar chinese stick gauges are wildly innacurate.
And difficult to use.
The quality digitals have a clip that locks onto the valve stem and gives instant readout and it becomes a one handed process.
Lock the hose on the valve stem, hold the gauge in your hand, push the bleed with your thumb.
Every tire ends up at exactly the same pressure.
Never trust the pressure gauge at the gas station.
Our tires are so friggen expensive that buying a good tire gauge is just protecting your investmant.