Frame resonance.

[Pinger]

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.

 

[b454rat]

I've never heard of that before, but I tend to think it's a Yukon/Tahoe/Suburban thing with all the cab space. I've had loud exhausts on trucks, with no reverb or droning, but I had a couple on Tahoe's and Yukon's and it's pretty noticeable. I had to take one off cause it was so bad. Rotating tires and changing pressure will effect tire noise, and gets amplified in the interior.

 

[jess t]

I bought a 2005 GMC C1500 new. It had the (they called it "frame beaming") vibration from the day I drove it off the lot. After many many useless attempts to fix it, they (field office?) told me it was a normal occurrence and "no repair is necessary", basically "pound sand". Never another GMC product. My .02 Jess

 

[SUBURBAN5]

Do you have any play in the drive train? Theres a vibration section in the gm manual explaining diagnosing procedures

 

[Pinger]

Do you have any play in the drive train? Theres a vibration section in the gm manual explaining diagnosing procedures

No, I don't think so. When I'm underneath it I give the propshaft a tug to check for play in the UJs and they are fine. Other mounts are fine - as expected at less than 50,000 miles. The frequency seems high even for propshaft imbalance (which would be 4.11 times higher than wheel generated imbalance).
The other thing that makes me think it is 'frame beaming' is that the mild version of it at 60mph has gone - now it is more intrusive and at a different speed. What to do about is what has me foxed. Whether it comes good as the tyres settle into their new locations, or I jack up the tyre pressures again - or find some other F/R 'balance'. To describe how it feels - like driving over 'washboard' grooved concrete - that brrbuzzing.

 

[SUBURBAN5]

No, I don't think so. When I'm underneath it I give the propshaft a tug to check for play in the UJs and they are fine. Other mounts are fine - as expected at less than 50,000 miles. The frequency seems high even for propshaft imbalance (which would be 4.11 times higher than wheel generated imbalance).
The other thing that makes me think it is 'frame beaming' is that the mild version of it at 60mph has gone - now it is more intrusive and at a different speed. What to do about is what has me foxed. Whether it comes good as the tyres settle into their new locations, or I jack up the tyre pressures again - or find some other F/R 'balance'. To describe how it feels - like driving over 'washboard' grooved concrete - that brrbuzzing.

Download volume 1 and you'll see what testing to check. I also need to do this test but i shot the parts cannon instead. I can tell you I still have a vibration and it's annoying.... Not as severe as before though

 

[Orpedcrow]

What kind of tires are you running?

 

[GoToGuy]

Do you have an aggressive tread off road , snow type tire?
Yes the max inflation is 44 psi at MAXIMUM LOAD of XXXX.
If your not running at maximum gross loads , your over inflating. It must ride harshly, are your fillings getting loose yet? Drop the pressure to 35. Your hard ride and aggressive tread, high tire pressure could be finding a harmonic frequency equal to or on balance with frame, body.
It may not " feel right " but there is a recommended tire pressure on the door jam or in the handbook.

 

[Drunkcanuk]

Mounts, motor, tranny and transfer case, can cause vibration issues at certain speeds as well.

 

[Keeper]

Add some weight to the rig (2nd row peeps and/or add trailer) to help identify source? suspension or driveline

Increase frequency of tire rotations?