Auto Parts: auto transmission, power steering pump, auto transmission

Question
I have a 2001 Buick La Sabre auto transmission.
When it shifts into high gear about 40 or 50 miles/hr I have begun to hear a hum from under the hood, not the tires.
What do you think it could be.

Answer
    If the tone of the hum changes with the speed of the car, then there's most likely a problem with one of the axles.  There are a lot of things under the hood that could make a noise like this.  I wonder if you're only hearing it high gear because the engine is so much quieter in that gear.  I would try putting the car in third gear and driving it like that for a while.  If you still hear the noise then it's probably not the transmission.  I would be suspicious of one of the engine accessories: alternator, A/C compressor, or power steering pump.  Even a weak or broken engine mount could cause a noise like this.