Whining Pathfinder

Guys,

I have an R51 2007 Nissan Pathfinder, over the last 5 or 6K a whining sound has developed in the engine, not unlike the sound of a turbo chager when it cuts in, that high pitched sort of sound.

Now I have taken it to Nissan and they thought it was belt related, and possibly the belt tensioners, so I had them replaced, but no joy, they are now saying they aren't quite sure what the problem is.

So I took it to two of the most respected 4WD mechanics in Perth, and they are stumped as well. it doesn't seem to affect the driving performance of the vehicle, but i really would like to get it fixed before it becomes a major problem.

Has anyone else experienced a similar problem on this model, any thoughts would be appreciated.