My tire "sing" on the interstate. Some tread patterns will cause more noise than others, all terrains and off road tires tend to hum or sing mine where nearly silent for the first thousand miles or so but now they are pretty loud. Some performance tires will have a higher pitches noise. It could easily just be your tires.

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2005 X5 4.4i
-stock

2000 X5 4.4i
-off road ready