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It is well known that there are two versions of the MPE.
However, I think there is another issue that is effecting drone too.
Not all N55 engines are the same. Early 535i models feature a pneumatic wastegate (PWG) like the N54 models used. Then, starting with 07/2014 production, BMW switched the 535i to an Electronic wastegate (EWG) design.
I’d be willing to bet that the change in design also came with modified watergate programming/behavior in the ECU. This could easily account for why some people have drone and others not.
In addition, BMW adjusts various ECU parameters through software updates. These update could have made the drone more or less appearant over the years.
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