Air leak- try the soapy water troubleshooting method to locate the leak. If not at the bag trace the system connections back the the pump and go from there.

In some cases a fluid change will correct a transmission issue. It is more likely it will not. Cost of transmission repair obviously varies by shop. I suggest the first step is to go to a well recommended Indy shop that frequently repairs BMW transmissions for diagnosis rather than wade through speculation that will likely make you feel worse.

A 4.8 is a great choice based on what you have had in the past. To find one with only 80k miles and an extended warranty is good luck.

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