Mafain I think I am treading the same path too at 105,000 miles same gearbox....perhaps a coincidence but got both front tires replaced and as soon as drove off i could hear a sort of low key rumbling up fron't, put it down to the tire tread style being noisier.

After 2 mins of driving noise was gone, BUT since then and because I have driven car for good part of 14 years nearly, i know and feel when something is different. Its when I am in low gear in traffic nearly idling along and hit the pedal lightly i can only describe it as a slight rumble, then all good when I take off, i will check too if i have same symptoms as you when it is under load and on inclines....

Hmmm reading around a bit could maybe becos i have fitted other tires up front not exact same tread as rears....didnt know the xdrive was that sensitive

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N