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[Andy H]

The traditional cause of this kind of noise is soft engine mounts and a badly fitted exhaust that makes metal to metal contact under certain conditions. The noise often sounds like some major mechanical component about to need expensive attention.

The diff is mounted in a subframe with rubber mountings. The subframe has to be lowered to change the road springs. Soggy mountings or loose bolts might allow the diff to move around & make odd noises.

[damon80]

Cheers for the advice Andy - no, ain't tried that.  Will give it a go.

How much play is too much?  Because surely there should be a bit of play to allow for movement of suspension?

And correct me if I'm wrong - if it was a duffed diff, would the noise be constant, as oppose to the symptoms I'm getting?

You should just have a small amount of loading on the taper bearings on the input to the diff, and NO movement of the input shaft. It's unlikely that the diff will be shot so soon even if the input bearing is slightly slack. Have look at what moves on the suspension re the diff in put, the movement of the suspension doesn't have a bearing (no pun intended  ) on the diff input shaft. 

Jeez, what's that in English, Andy??!!  Bearing in mind that I'm somewhat spanner-shy 

If you remove the 3 bolts at the diff end of the prop shaft and swing it to one side you can see the nut holding the diff input flange in place. On my Senator this nut was loose, it shouldn't have been, but don't tighten any ,ore than a nip, otherwise you'll bugger the input shaft bearings up.     
Any better Damon?    

Yep, much better thanks Andy! 
I'll have a gander int morning.  Cheers bud

[damon80]

The traditional cause of this kind of noise is soft engine mounts and a badly fitted exhaust that makes metal to metal contact under certain conditions. The noise often sounds like some major mechanical component about to need expensive attention.

The diff is mounted in a subframe with rubber mountings. The subframe has to be lowered to change the road springs. Soggy mountings or loose bolts might allow the diff to move around & make odd noises.

Hmmm, now that's a thought...  I've noticed that if I "blip" the throttle when stationary, the car rocks from side to side - not a great deal, but much more noticable than in my last Omega.

Could this point to soft engine mounts maybe?

[Andy H]

1. I allowed my local garage to replace the pinion oil seal on my carlton (Omega A)(very similar diff with crush spacer between input shaft bearings). The diff was scrap after 30 miles because they did the nut up until it felt 'tight'. The input spider is on extremely tight splines, the nut is a self locking nut. If the nut has come loose you need to look for the reason why.

2. Try moving your tail pipe around with the engine ticking over, are all the rubber mounts in position and in good condition? has the exhaust system broken in two pieces? There are so many supports that a broken exhaust won't fall off but it will make strange noises. Does the exhaust make contact with parts of the suspension or body if you bounce it up and down?

[Andy H]

The traditional cause of this kind of noise is soft engine mounts and a badly fitted exhaust that makes metal to metal contact under certain conditions. The noise often sounds like some major mechanical component about to need expensive attention.

The diff is mounted in a subframe with rubber mountings. The subframe has to be lowered to change the road springs. Soggy mountings or loose bolts might allow the diff to move around & make odd noises.

Hmmm, now that's a thought...  I've noticed that if I "blip" the throttle when stationary, the car rocks from side to side - not a great deal, but much more noticable than in my last Omega.

Could this point to soft engine mounts maybe?

Sorry, I wasn't trying to suggest that your engine had soft engine mounts, I was trying to draw attention to the ways in which rubber mounted items can make alarming noises if you get metal to metal contact.

[Andy B]

..... the nut is a self locking nut. If the nut has come loose you need to look for the reason why. ......

It's a long time ago now,  but I'm sure that my Senator's wasn't a lock nut.  :-/ Possibly it should have been, it would make sence to me if it had been. I used a squirt of Locktite on my nut!     

[3.2 manual]

what about propshaft whirl? the centre bearing on the shaft, dont know if this is a problem or even present on these, then but my carlton had one go at about 120000 miles