Maybe it’s the air purge system on top of the radiator. It has a jiggle valve installed the wrong way at the filler neck.
I run 5-50 full Castrol synthetic, No change in the moaning.
Yes, any diaphragm of air, fuel, water, etc. where the valve is cold or the air or fluid is cold and the valve has a hard time closing for a few seconds
Senior moment there Janusz, I wanted to say RIGHT side… where the carbon canister is.
I’m pretty sure mine does not have the valve, never did… unless I haven’t noticed? Better go and check!
Gregmatic … ??? 
Not sure what a moose sounds like, but I don’t think I hear it when I start up my car. Lots of other squeaks, squeals and clattering, but no moose.
Has anyone outside of the US reported this? Maybe it is something to do with US emissions controls.
I’m in Canada and hear it on my '89 and if I remember correctly I heard in on my '85 when I had it as well. I always assumed it was the oil system purging air as per the TSB. Thought for a while it might be cooling system related… but… And if it matters I run synthetic 5w50. Similar noises when I ran regular 20w50.
Having said that, US and Canadian emissions standards are pretty similar.
My understanding is that early cars had bypass oil cooling for all markets; HE for Germany had full flow, while rest of world (including US, Canada, Australia) kept the bypass oil cooler. Then the 6.0L had full flow for all markets. Don’t know years this applies to.
Now you got me curious about something … That pair of “banjo” bolts in the oil feed system … is THAT what the holes in them are for (i.e. air purging)?
Do you mean the banjo bolts on the back of the tappet blocks (one on each side essentially at the firewall)?
If so, then no they are not specifically for purging air, those are the oil supply.
I still think it’s the pressure relief valve making that noise. Cold oil, high pressure. (My aftermarket oil pressure gauge reads 75psi at start up, which I think is when the relief valve kicks in?)
Within a minute or two, it settles down to 60-75psi, and the noise is gone.
And perhaps it makes sense that I can hear it better inside the car than outside. If I’m not mistaken, the valve is located around the oil filter housing? Which sits just in front of the driver footwell?
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I used to have a v12 years ago and recall a sound like the one described on starting. It was like it for years and I think it went after I replaced the timing chain and very worn tensioners when it got more rattly. The fibre tensioner just disintegrated when I took it out.
this is closer. But luckily my engine doesn’t sound this bad!
k. But why the holes in them, then? And supposedly, like with the radiator banjo bolt, the hole is misaligned by Jag, requiring aftermarket replacement with ones that have them in the correct spot. 
A picture here is worth lots 'o words. Here are the improved oil feed banjo bolts, ready to be installed in my V12.
As you can see, there is a passage machined axially through the middle of the bolt, and there is another passage machined radially through the bolt, near the head. This allows oil to flow UP from the supply pipe, which fastens AROUND the radially machined passage, and then flow axially through the bolt, and into the engine tappet block. Thus suppling oil to the valve train.
I don’t have a picture handy of the oil feed pipework because its buried behind the motor, but for reference here is the air bleed banjo from the top of the radiator.
The problem with the original jag banjo fittings is the groove that is machined in the banjo fitting on the supply side doesn’t line up very well with that radially machined passage. Thus restricting oil flow and perhaps even cutting it off if some crud were allowed up build up!!
Yes, that’s them. I’m just hoping I’m not starving oil somewhere in the system b/c it has the OEM bolts still in the car. Is it a major chore to r/r them, btw?
No. It helps though if you have small hands cause it is pretty cramped space. And IIRC there are some good “been there done that” tips in the book and in the archives. You don’t want to drop anything.
Doing them with everything in place will be exceedingly fiddly, but probably, doable.
When I had my intake manifolds off to do cam cover gaskets and half moon seals I was going to replace them (banjo bolts) as access was MUCH improved. However I found the existing banjo’s to be exceedingly tight, and was afraid if I applied any more torque I would snap one, and that would require engine removal to remedy.
So this winter the engine and trans are coming out anyways, so I’ll do them then.