Help!! Drivetrain issue
 

It started having vibration over 70mph. It gets bad enough to vibrate the rear view mirror at around 78mph. It then lessons a bit but still is very noticeable above 80mph. The vibration is still there when I put the car in neutral between 70 to 80 mph. I also have notice a thumping sound under moderate acceleration (around 5 thumps a second) and stops when I release the gas. Obviously feels like it is in the drive train. The car is not losing traction or even close to losing traction when accelerating. No other unusual noises in any other driving condition. The car is as in my sig but has stock 3.45s in it. What do you think?

Typically highway vibrations are due to tire/wheel balance issues. When you shifted into neutral you effectively took the drive train out of the picture unless it's a wheel bearing or u-joint issue. Any noises at slow speeds like a creak or squeak that sounds rotational?

Paul

I do have some type of suspension squeak I hear when I go over bumps. Otherwise nothing else. Nothing rotational as far as I can tell.

Can you tell where the noise is coming from, inside, outside, front, rear, left or right?

Paul

Sounds like underneath and around the back end of the car. The thump I can feel in the seat and on the arm rest and also sounds like coming from the rear of the car.

Is it a squeak or a thump?

When under moderate acceleration, very distinct thumping that you can even feel. The squeak I was talking about going over bumps almost sounds like a squeaking bed with springs.

The squeak could be the stabilizer links but the thumping might be a tire or wheel. I would start with a road force balance of the wheels and tires then while it's up in the air check the links to see if any of them are loose.

Does it matter if you have a passenger in the front seat?

Paul

No matter with the passenger. I just went out again to try a little more spirited driving. The thump goes about 4 to 5 times and no more thump while still accelerating. It will come back again if I let off and then re accelerate. I was thinking about the balance. I got my last tires from Discount Tires, so I have lifetime balance. Definitely a good start for the vibration.

Acceleration usually doesn't have anything to do with a wheel vibration unless your reaching the vibration speed. I would definitely start with a road force balance then see if that takes care of it. While it's up in the air take a look at the drive shaft to any marks or dents, if there are any that could be your issue also.

Good Luck,
Paul

Thanks Paul. I will let you know what it turns out to be. Hopefully by the end of the week I will have an answer. Thanks again!

I'd check for loose, bent, or broken bolts at your driveshaft/differential input shaft coupler "donut".

Second the driveshaft. A bad universal joint will show similar symptoms. Does it diminish if the clutch pedal is pushed in (or you shift to neutral) at highway speed?

I had the same issue with my Trans Am. I thought that when I put it in neutral it was still there. Found out it was the driveshaft. I thought that even with it in neutral at 70 it would stop the vibration.....not true. It was a bad u joint on the input shaft