Expansion tank.

Replace it along with the hoses, Tstat, and fan switch if it hasn't been done. You also could elect to do Water pump at same time if you choose to. Only go OE.

BMW E53 X5 M54 3.0L Engine Cooling - 17117573781OEKT - Expansion Tank Replacement Kit - ES#2588202

That is the expansion tank kit, you could also get the full cooling kit, as the hoses likely are brittle and need replacing too:

BMW E53 X5 M54 3.0L Engine Cooling - ECSE531711-1 - ECS Cooling System Refresh Kit - Level 1 - ES#2158099

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards