"whirring" sound when slowing down at 40 kmh...

The latest in a long and inglorious list of problems with my '99 2.0 TDi lately is this:
When im driving (uphill, downhill, straight and level), and start to slow down in second or third gear, there is a real "whirring" sound at exactly 40 kmh - at 45 or 35 kmh nothing and also nothing when accelerating
It seems to be coming from the engine / gearbox? (front end!)
it lasts as long as i am at the 40 kmh mark and stops instantly when travelling slower or faster than this.
What could be the problem here? gearbox? IRD? tyres??
If I slowdown "free wheeling" then all is quiet and as should be.
Does anyone have any ideas

Sounds like tyres, mine whines like a army landy on the camacs fitted and allways at thirty mph
took them off yesterday (its summer now) peace has been restored which is good as I was begining to think the noise was the diff!

Cheers Edge,
Im in Innsbruck so I have michelin winter sport (chunky) on 16" alloys for another month (winter tyres by law!) and then its back to the summer 17" wheels with pirelli low profile scorpions.
Do you think "over" or "under" inflating might help a bit?
I dont want to scoobi the tyres, im just getting fed up of being asked "what was that?" when slowing down!!

don't know about under inflating the tyres she handles like a run away camel on soft sand as it is