Alignment is the key factor but I don't know what settings you had.
I spoke earlier about replacing the front bush on the diff of my Q4. This surprisingly was less expensive than originally thought, £22 plus VAT. It was no trouble to change, indeed it took very little time to pull out (aluminium body) and to re-fit. The reason I changed it was principally because of a low thud on taking up drive in first and progressing to second. It was non metallic in nature, but thought there must be some association with the bush mountings at 102,000 miles.
Anyway, the bush has been changed and it is true, the old one was not visibly worn. However, the surface of the bush had gone matt black. The new-mould sheen had gone from the surface. Moreover, it was only marginally less hard than the new one. But the colour was indicative of the rubber of a totally wrecked bush.
Having fitted the new bush, I then went for a drive. The problem has now shifted to all gears. It may be that it was always there but better damped in 3,4,5 & sixth by the old bush. However, it now occurs also on dis-engaging the drive to change gear.
We have since reviewed our work and a couple of issues have arisen.
1) Whilst the front bush was being replaced, the diff was hanging on the two rear mounting bushes and although unable to get at those bushes, the flex seemed too much for the work they have to do.
2) If they are bolted through then visual evidence suggests they have deteriorated, not to say collapsed. But I believe the bolts are 12mm., so that, I feel, should have restricted the amount to which they allowed the diff to drop; droop.
3) The front diff mounting bracket is substantial and fixed to the floor pan by four 12mm. bolts. The drillings are oblong, suggesting there is an alignment issue with regard to the front bush and hence the location of the prop shaft and diff input flange.
I have complained to Paul that I am not happy with the 159 exhaust note as it seems more like a TVR V8, giving a constant low roar. Revving the engine when stationary, it is typically "Crackly", and Alfa - like.
I think you may well have twigged where I am going with this, but under these circumstances, I find it best to postulate in print, in the hope that someone may contribute something I may have missed. However, this is where I am going.
a), Under drive conditions, the prop shaft will demonstrate a gyroscopic character always trying to assume a central rotational axis - "Continuously".
b), In doing so, it effectively is generating a low frequency resonance as it perpetually tries to assume its true rotational axis.
c), The axis between the prop shaft and the diff should be absolutely at right angles with the axis between the diff output shafts and the drive shafts to the rear wheels.
d), If the front diff bush mounting bracket is not set to ensure this is the case, then the stresses, due to rotational/gyroscopic action will be transferred to the differential output shafts and cause the rear diff mounting bushes to weaken.
e), It would, I think, also be responsible for the almost "Powder - coating", bead-like matt surface of the old bush, although ostensibly I cannot say the bush has failed in the classic sense.
f), Prior to taking up drive, the prop shaft/differential input shaft are in their static rest locations as are the output shafts.
g) On applying drive, the rotational gyroscopic forces of the prop shaft are attempting to re-align it in its true rotational axis.
h), This is also true when dis-engaging the clutch and then re-engaging it when changing gear. This is borne out by virtue of the fact this low thud now occurs not just in taking up drive, but also, dis-engaging the clutch.
Had there been no change in the conditions after the bush was replaced, then this would indicate the problem lay elsewhere. That there has been a change, indicates a strong link between the front diff mounting bracket and the overall alignment of the propeller shaft, the differential unit and the drive shafts to the rear wheels. I believe, I am faced with the need to replace the rear mounting bushes as well and doing dynamic alignment of the front mounting bush bracket to try to minimize the run-out (of true) of the prop shaft, with respect to the differential location, adjusting the bracket accordingly to find the ideal position. I think, when this is done, the low frequency TVR note will reduce, as will the rear-end thunk, which I currently believe this mis-alignment is causing. Your thoughts on this matter would be appreciated and if per-chance you know of any company with experience in this area of alignment, I would appreciate a name.