2006 LT AWD 107k miles

This issue came on fast (within a day or two) and with little warning. PLEASE NOTE: the mph given are exact and consistent

SYMPTOMS

- shifts normally(or appears to shift normally)
- accelerates normally
- drives under 30 smooth and normally
- consistently (meaning all the time) as soon as the speed hits 30 mph a faint rumble begins(it feels like its coming from behind)
- By 35 MPH it feels like someone is jackhammering under my vehicle
- Rumble becomes less severe at around 55 and is barely noticeable at 60+
- Rumble appears to be affected by bumps in the road as when I hit a bump the rumble INCREASES momentarily when the car "bottoms out" on the bump
- Conversely, the rumble subsides momentarily when the car "tops out" from a bump
- Appears teh rumble is affected by vertical G forces
- As soon as the car slows to under 35 the rumble shuts off like if someone turned a switch off and does not return again until 35 mph.

ACTIONS IVE TAKEN TO DIAGNOSE

- being on or off the gas has no perceived effect
- shifting to Neutral while the rumble is occurring has no perceived effect
- steering has no effect on the rumble
- Applying the brake has no effect

ACTIONS NOT TAKEN

- have NOT checked the trans fluid due the the ridiculous and stupid location/access to the trans dipstick
- have not changed any of the fluids of the drivetrain (trans or front/rear differential)

OUTCOME

TBD and hopefully someone has experienced this and can steer me towards a REAL fix instead of a goose hunt

have you had anyone check the center bearing in the driveshaft?

If it stops when the vehicle is at the top of a bump, it sounds like a suspension/steering linkage issue to me.

Sound to me like a wheel bearing. They will make a varying sound.

Possibly. Easy enough to check.

Yeah - but tough to replace !

I've never done one on a modern car, they are easy on a Studebaker with the right tools.

Yep - things have probably changed a "little" in the past 60 years ! Don't look for a ball bearing race behind a grease seal.

HEY!! It's ONLY 52 years since they made Studebakers!

Yeah - maybe an Avanti.

Funny story- When I was young, I was walking past a neighbor's car - a Studebaker with a wooden running board, and it must have had a large splinter sticking out, and it pierced my thigh, and I still have the scar - probably 65 years later, because I must have been real short - 2-3 years old ! Weird, huh ?

Roommate in college drove a Golden Hawk. Neat car.