having trouble when the car goes into 4th. it comes it at about 70k's and at lower revs (say 1200-1400) i get this big whirring vibration. it kind of pulses slowly but pisses off once the revs get a bit higher and i'm going a bit faster. anyone else had this problem? am i describing it well enough? thanks.

if it still does it at the speed your saying in neutral its the drive shaft out of balance or worn universal joints. Check for any missing balance weights from the ends of the shaft, also check for play in the universal joints.

i've kept it in third at the same speed and there's no prob. neutral wouldn't be any different would it? i'll check it out anyway.

Putting it into neutral at the speed where the vibration is help to find out if its engine revs, transmission or driveshaft at fault.
If the problem exists in neutral, third and drive its a balance issue with the driveshaft. its either out of balance due to worn universal joints, shaft missing balance weights, shaft damaged or bent.
Check for worn UJ's by looking for signs of black powder around the bearing seals, check for play in the bearings by twisting the shaft back and forth whilst handbrake on and in park. look for missing balance weights at either end of the shaft . The vibration will normally be worse when decelerating at speeds over approx 80km/h.

might not even be driveline related - could be a bulging tyre / tyre lost it's balancing weights etc.

try locking it down a gear, pulling the same revs and see if it vibrates then.

does anyone know if a failing TC lock would do this? cause dosen't it lock at around that speed?

have seen a few where the planatry gear set wear and make that noise although is rare but can happen

it was found that the au had a frequency of around 700Hz being emited from the transmission it was fixed by fitting a damper to the slip yoke of the drive shaft that absorbs this sound. i built a shaft last week with one on and its a whole different car now. its an el fairmont ghia.

i have a sad feeling it's the gearbox. i think i'll have to have someone check it out properly for me though. it ONLY happens in 4th. not 3rd or in neutral. why must every car i buy fall apart?!

as i said some models needed a damper fitted to the shaft to absorb any vibrations emmited from the trans or diff, these vibrations are not balance related.

i'll have to have someone look at it for me. thanks though fellas.

Sounds like a torsional vibration problem. Very very common.

Torsional vibration will only occur generally in only 1 or possibly 2 specific conditions, and if you go 10% or more either side of the point where you have torsional vibration, it will completely dissapear.

Get it looked at quick though, and fixed. Torsional vibration is extremely damaging. Don't drive where the vibration occurs untill you get it fixed. Try to stay 10% either side. Whether that means selecting 3rd, driving slow or speeding.

If you don't fix it, it will break shafts. In my line of work we deal with agricultural gearboxes and I've seen 3" diameter shafts made out of solid 4140 (a grade of high tensile steel) broken in 2 by torsional vibration. It is easily cured though.

Sorry to be so blunt, but if it is torsional vibration, and I strongly suspect it is, there's no point telling you everything will be ok, cause it won't unless you get it cured.

so what's the cure? and what do u reckon it'll cost? dare i ask.

and what exactly is torsional vibration?

The cure is a dampener of some sort. I'm not sure how much it would cost in a car.

Now, as for what torsional vibration is:
Torsional vibration is related to torque, hence the term torsional. Basically what it is is every time a cylinder fires and rotates the engine clockwise (looking at the engine from the front) there is a slight kickback in the opposite direction. This happens in EVERY setup with an internal combustion engine.

Now, this kickback delivers an amount of torque through the drive train and manifests itself in the power transmission components. To help you understand what it is, think of kicking a footy. It goes 50M, then bounces on the ground. The bouncing of the ball is the torsional vibration.

So, in that analogy, your foot is the engine. Note that the ball doesn't bounce on your foot, or in mid air. Only when it hits the ground. This represents the gearbox, hence why torsional vibration manifests itself in the power transmission components.

Ok, so I said before that this kick back has a specific amount of torque. If you've seen a power graph of an engine, you'll know that the torque curve is a nice flat line that starts high about 2000RPM and gets lower and lower as the engine speeds up. The torsional vibration also has a torque curve. It's a lumpy line that sits underneath (i.e lower torque) the engine torque line. As long at the torsional vibration torque line is below the engine torque line, you won't get vibration cause the engine torque is over-powering the torsional vibration torque. So let's say that @ 1700RPM @ 70km/h you get this torsional vibration. Let's say that the engine is producing 100 foot pounds of torque, but the torsional vibration torque is 130 foot pounds. The engine torque line is no longer above the torsional vibration torque line, so you get the vibration. Now, if you go up a hill @ the same speed, and now the engine has to produce 140 foot pounds, the torsional vibration will go away.

So, back to the analogy of the football, torsional vibration is when the football bounced higher that you kicked it, i.e it bounces 60M up into the air when you only kicked it up 40M into the air.

Torsional vibration is almost always a spike, occurs under one set of conditions only, and is cause by an unfortunate mix of inertia, shaft ductility, (i.e how much it twists and flexes) and a few other factors.

I hope you can understand that.

no, but thanks anyway. it sounds like the problem.