High pitch noise

I have a 2004 Yukon Denali that I purchased new a couple of months ago. Since the first day I have had it when I am cruising around 45 mph or more there is a high pitch noise that can be heard. It is especially apperent when driving under a bridge. I cannot figure it out. The dealer looked at it and could not find anything. Does anyone els have this problem? It almost sounds like wind noise from the mirrors. Any help would be appreciated.

could it be the awd system?

Am I crazy? I just read that the fuel pump can whistle while driving. Is this true?

This TSB refers to Drivetrain - Whining Noise from Rear Axle

Thanks for the reply. Are you saying it is the rear axle making this noise? Is there a fix for this problem? That reminds me that my Denali was short 3 bottles of rear axle fluid when I bought it. I dont know why I checked but I did and it was low.