OK, for background: 2008 3LT Z51, 92k miles, headers & cat-delete X mids, stock tail with NPP. Wheels are aftermarket but stock sizes. Everything else is stock. Not sure if all of that is germane to the issue but didn't want to leave anything out.

So, starting around 60mph and getting increasingly worse at higher speeds is this very rhythmic vibration that I cannot find the source of. It is a low-speed vibration, no sound (that I can hear). I can feel it in the floorboard and seat, and the steering wheel twitches with a light touch, but it cannot be seen or even felt through the center console or dash. It is very similar to a wheel out of balance, only "heavier" and it comes and goes very regularly. It happens approximately 1/sec: nothing for a second, vibration for maybe half a second, then nothing for a second, and the space/time between vibrations does not seem to change with speed but the intensity does. Also, this does not happen all the time. Some days it's barely noticeable and other days it's in my face.

My first thought was wheel balance, but I've had that before and it doesn't feel like this. It's almost like a flat spot that comes and goes, only you can't feel it past the floorboards and seats.

Tire wear looks good, suspension is solid, mechanically she seems sound. This is my attempt to pre-diagnose before heading to the shop.

 

I'm gonna take some shots in the dark.

Wheel hubs are original?
Bushings original?
Tie rods original?

Is it consistent on the same roads? Like if you drive down a street, does it always do it if you have the same speed, or only sometimes? If it doesn't replicate on the same road and the same speed... Probably gonna be tough to diagnose unless you get lucky and it's something under the car that's flapping around. Might be able to see that if you jack it up.

 

Not 100% sure. I've had the car inspected by my local Vette shop, they did not mention these things to me.

That's the crux, really. it's my daily and I commute. Same freeway, pretty much same lane strategy every day and the issue is not 100% consistent: some days I don't even notice it and some days I think I should pull over. I don't think anything is flapping underneath; that would be a consistent thing, not the come and go (I would guess - nothing is certain at this point).

 

Some more shots in the dark... Maybe a warped rotor. Early torque tube failures will have some similar symptoms but you don't hear any noise.

 

Manual or auto ? Does it do it in 4th,6th and 6th gears at the same road speed but different RPM ?

 

I've had the exact same thing happen on different car,,, this is an easy free check ... Look at your wheel weights, I've had a wheel/tire be balanced but they used way too much weights on the wheel, it needed to be indexed, break the tire down, rotate the tire on the rim, re-inflate and then rebalance with very little weight, if you have a wheel with lots of weights on it that's a good place to start!