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It may also be a mid pipe (or rear axle back junction) gasket replacement?
I had this issue before, and it would get loud upon revving. Noticing the third video, it seems like it did get louder.
And, it kind of resembles the sound a steam engine -- but a really small one that compressing air?
Either way, have them check the seals for the exhaust system. If you're the 2nd owner, then the previous one might have given the vehicle an exhaust upgrade temporarily before selling the vehicle to you (or back into BMW at lease end).
Cheers.
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