One way is a soapy water solution. Air leaking will cause bubbles that are visible. I would start with air bags, then air lines. If you don't find the leak next stop is the compressor where seals can fail. Another way is to wait until the leak get worse so it is easier to find.

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