My 07 X5 4.8 also come with Dunlop 20" and on certain road surface, I do notice that hollow roaring noise like described above.

That roaring noise seem to be more prevalent with higher tire pressure (in the case when my cold tire pressure coming from the dealer was at 35psi front and 37psi rear). I kept my cold tire pressure at 32psi front and 34 psi rear now and seem less noisy (still happens on some road surface just not as loud)

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13 X5M