It doesn't sound like the torque convertor--usually a grinding/whining noise or vibration at certain speeds but no slipping is a torque convertor symptom.

Fluid could be low and it won't hurt to change it but once the transmission is showing failure symptoms a fluid change less apt to be the solution.

Your symptoms sound more like an internal problem--which does not have to be something that means a total rebuild but if that is the case that's what I would if it is torn down anyway. Internal issues don't have to break the bank.

Once you get the problem corrected I suggest adding Dinan transmission software that will make the transmission shift faster and firmer and carry a higher rpm in each gear. I think it contributes to longevity. I have a 4.6 and have many engine mods and at 110,000 mi. no issues.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas