A buddy of mine's 02 WS6 makes a whoosing (fluidic like) sound once he hits 10 mph. It goes away when he slows down past 10mph or if he stays below it.

When he drives around normally the sound remains. When he went WOT, the sound completly disappears. It doesn't make the sound when he's in neutral and revs it.

It sounds like it's comming from the rear end. He flushed his fluid out a week ago.

Any ideas?? I guessed it was the torque convertor not locking up properly??