Similar sounds have been associated with stripped out splines on the front driveshaft and/or stretched transfer case chain. Have someone walk along next to the car to try and narrow down the location of the sound. I don't think you can tell very well from inside the car.

Option #2 is the rear subframe bushings. They generate a rear clunk when worn. There are threads here and a couple of people/places you can rent the special tool to pull the old bushings out to replace them. It's not too big a job - a few hours with the special tool.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles