old_school,
May sound silly but I'll ask anyway.
If the gearbox is pointing down at the back, i'e engine + gearbox are sloping down towards the back of the car. IF this is the case the diff pinion must be pointing up. Remembering the angles are relative. So, to make the diff pinion account for acceleration you should mount it 2 deg twisted down from being parallel with the gearbox angle. Don't laugh, I've got this wrong in the past.

If your issue is tracked to you pinion angle, from what you have described below you have too much angle as the noise is evident under coast and decel (assuming the angle is in the correct direction). Under Decel the pinion is being twisted anticlockwise (Looking from the passenger side i.e forcing down) which will increase the angle difference. Under Accel it is being twisted clockwise and closes the angle difference (This is what Caltrac/traction rods try to stop, the upwards twisting of the diff which causes leaf spring bind/twisting).

Another issue I have seen with the way CAs mount into 1200 is generally you do as described above due to the space in the trans tunnel, this then pushes gearbox oil towards the rear section of the gearbox and it sometimes comes out past the rear oil seal under acceleration(Yoke area). As we all know, CA love to feed excessive oil to the heads and I have always had issues with the rear of the passenger rocker cover leaking (due to the rear slope of the engine and CA head oil feed).

I've made some pretty heavy changes to the trans tunnel to get the gearbox to sit higher, thereby decreasing the rearward slop.

My 2c..

The pinion angle should be the same as the engine/gearbox angle.

It doesn't matter what the driveshaft angle is as long as the pinion angle matches the longitudinal crank angle, that way the unis at each end of hte shaft are in plane and cancel each other out.

Also, have you checked the driveshaft?
Are the unis in line with each other?

did some quick reading on uni's the other day when i first read this....

angle doesn't matter regarding keeping a constant output velocity, a single uni produces a sinusoidal velocity (varies up and down like a wave pattern) but the fact there is two uni's... one at each end of the shaft... means that this effect is cancelled out by the second uni... and therefore you already have a constant velocity

i assume that by reducing the angles you reduce the energy lost in translating the torque through the uni... therefore you are just trying to minimise drivetrain energy loss? is that the aim? or are we trying to achieve a constant velocity?

1200rallycar,
Not trying to target you but from my understanding this is incorrect.
The reason is the relative radius which is used to determine the amplitude of the sinusoidal angular velocity is a function of uni operating angle. therefore if the front and rear uni's are not very closely aligned the do not completely cancel the wave out.
Wiki explains it better then what I can, see double Cardan Shaft for explination
Wiki
ssskiwi, Agreed, although there is a recommend maximum operating angle of most uni joints.
Anyway Ive given more then my 2c on this topic. I hope it helps old_school diagnose his issues.

Yeah thanks mcgee, Gearbox points slightly down and gearbox points slightly up.

Might look at putting some shims or wedges in to play with the angle slightly to see what effect this has.

Yes universals are aligned correctly on the shaft and they should cancel each other out. But at end of the day I was guessing the required difference in pinion angle to get the pinion and gearbox output parallel under load.

Its just frustrating having spent so much time on this car and still not getting it 100% sorted.

Cheers for your input guys

It's been a good thread. I need to take a closer look at mine now.. I was just blaming B&W for the noises