Frame resonance.

[Hipster]

I have had so many 19th birthdays, I don't think I am ever going to turn 20. Even my kids are older than me. But who really wants to grow up?

32nd anniversary of my 21st. Wish knew then what I know now. lol

 

[Caman96]

It’s tough being in your late 50’s. Much better in my early 60’s!

 

[Martin Evans]

Radials were a new thing when I learned. lol

Yeah, radials had just recently come to market when I started driving. I worked at a gas station when I was 15 and bias ply tires were still the norm. My brother and a friend of mine got together and bought me a set of US Indy wheels for Christmas for my '73 Duster back in the early '80s and I put some D70-14s on the front and F60-14s on the back. It was recommended that radials were to rotate in the same direction throughout their life back then. Now, it doesn't matter. When I've changed from summer to winter tires or vice-versa, I've just put the pair with the best tread on my drive wheels (front) without regard as to which side the came off of and never had an incident

 

[Martin Evans]

32nd anniversary of my 21st. Wish knew then what I know now. lol

You're just a pup. This is the 40th year that I have been of legal drinking age. Also, legal drinking age was 19 at the time.

 

[Martin Evans]

It’s tough being in your late 50’s. Much better in my early 60’s!

I turn 60 in 6 months and 5 days. Mark your calendars!

 

[tinfoil_hat]

To describe how it feels - like driving over 'washboard' grooved concrete

My old truck felt exactly like this when the trans started going out, right when the torque converter locked up. An experiment: When the vibration comes on, maintain speed and use your left foot on the brake pedal just enough to turn the lights on, not enough to engage the brake. This unlocks the TC.

 

[Pinger]

My old truck felt exactly like this when the trans started going out, right when the torque converter locked up. An experiment: When the vibration comes on, maintain speed and use your left foot on the brake pedal just enough to turn the lights on, not enough to engage the brake. This unlocks the TC.

A failing 4L-80E that's never towed and hasn't yet done 50,000 miles - I hope not! Propshaft though still needs checking out to eliminate.
There is one other change (apart from the application of 40lbs of underseal to the frame) and that's a new exhaust. Not that I suspect the exhaust itself - there's no resonance at the same engine speed in the lower gears - but it is a mass hung on a spring (frame) and a markedly lower mass compared to the old dual system and with different attachment points.

 

[Hipster]

A failing 4L-80E that's never towed and hasn't yet done 50,000 miles - I hope not! Propshaft though still needs checking out to eliminate.
There is one other change (apart from the application of 40lbs of underseal to the frame) and that's a new exhaust. Not that I suspect the exhaust itself - there's no resonance at the same engine speed in the lower gears - but it is a mass hung on a spring (frame) and a markedly lower mass compared to the old dual system and with different attachment points.

its 40llbs of mass to the frame. It's not going to change much resonance on a stationary component like the frame. My money still on something else is the issue. Vibration tends to accelerate wear, not always on the component vibrating. Somethings changed with tire rotation, wheel runout, tire runout, balance is a bit suspect. Along with something else thst might be purely coincidental.

 

[GrimsterGMC]

Have you tried returning the tires to their original positions to remove them from the equation? Maybe when you put one of the wheels back on it didn't seat on the hub properly.

 

[Drunkcanuk]

Is the vibration consistent at speed regardless of rpm?
Like does the vibration go away if you shift down a gear but stay at the same speed?