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May be a small leak in the AC system. That would explain why it gradually quiets when the engine is off. AC system is not building pressure when the engine is off, so all pressure leaks out and eventually equals out.
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2016 X3 35i |M-Sport|Megan Racing Coilovers|FBO+|BM3|Jordan Tuned|Valvetronic Designs|
1998 750iL V12|2017 440i|2019 991.2 GT3RS Weissach|2019 Model X|...and some others
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