Whirring noise from 406 Auto Gearbox when cold?

Hi,
I have a 1997 406 3.0 SE Coupe with auto box and just clocked-up 200k with no real problems until recently (but read on).
Started to get a whirring noise when starting the engine which seems
to be somehow related to the viscous clutch on the automatic gearbox. This noise gradually settles down after driving a few miles (i.e. the gearbox has warmed up). It's been gradually getting worse (louder, more evident) over the last few weeks. Noise is directly proportional to engine revs and appears in all gears including Park and Neutral.
The whirring noise reappears when the engine is heavily loaded (i.e. accelerating hard) or when braking heavily (the auto box will change down to assist engine braking). Noise disappears again when driving normally/steadily.
I've also noticed that when cold, sometimes when slowing down/braking the auto box will disengage - the engine revs drop to tickover even though I'm probably still doing 30-40 mph. I've also now realised this has been happening from new - every now and again when cold the auto- box would suddenly produce a big 'thunk' when slowing down and a consequential jolt in the car - basically the auto box was disengaging and then kicking back in to create the jolt. Again it only ever happened when cold.
Otherwise the car drives fine.
Obviously the car's getting a bit worn now, so I think there's something related to the viscous clutch. But I don't really know much about the internals on an auto box!
Dealer has run a diagnostic test on the box and found no problem - apart from low gearbox oil. However that's now all topped-up and there's no change. I don't really want to take it back to the dealer - they're really saying the only thing they can do is strip the whole gearbox down and look to see what the fault is - my confidence level in them finding something wrong and fixing it though is not high!
Suggestions anyone????
Cheers, Richard.
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changing the gearbox oil to full sythetic oil such as Castrol Trans Z may help. I've found Peugeot auto are fuzzy on oil and the semisynthetic oil specified doesn't seem work very well. It's worth a trial, a lot cheaper than stripping the gearbox down.

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