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Originally Posted by Ishniknork
From your Bimmerfest post:
"The Kumhos are 27.3" OD front and 27" OD rear. Could not find Revolutions per mile on this tire anywhere in the specs. The whine sounds like gear whine, short duration for a few seconds at a time and could be present at any given time and speed. Just from standstill or going 65 on the highway."
I'm no tire specialist nor XDrive expert but I think this might have been the problem. As the tires are not the same diameter it may be having an affect in the XDrive system because even going in a straight line the front and rear sets would be rotating at different rates (or Revolutions/Mile) all the time.
You also stated that when you put on identical Michlins the problem went away.
Maybe a BMW mechanic familiar with the XDrive system (say at your dealer) might be able to verify if that's the problem.
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I took that into consideration when buying Pirellis. The 245/45/18 front, 275/40/18 rears are identical 26.7" in diameter and identical 780.4 revolutions per mile. Still the intermittent X-drive problem (noise) persists. All I can think of is that the tires are rotation at slightly higher rpm. About 2.7% higher that the car thinks they should be rotating.