Auto change 'grunt'

[Rik]

I have noticed over the last couple of months that when my 2006 3.0d auto changes up - only into fourth gear I think - it makes a bit of a 'grunt' especially if accelerating with reasonable vigor. Nothing noticed on other changes.
Anyone had anything similar ? It this notification of impending doom ?

[Rik]

Took it into the dealers today and much to my surprise they knew of the problem. It occurs when going into or out of third gear and requires a BMW gearbox additive which will be obtained and added on tuesday. A known but fairly rare intermittent fault apparantly.