BMW Questions? Ask a BMW Repair Expert for Answers.
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I would recommend removing the belts and running the car briefly with the belts off to make sure the issue is not one of the belt driven components.
next would be revving the car to 4000 rpm in neutral and holding it there for a few min. if the noise goes away then this points to the lifters as the cause.
from there I would use a mechanic's stethoscope to try and further pinpoint the source.
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