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Your best bet when searching for answers is to use Google's site-specific search function; that is, type your search terms followed by "site:brickboard.com" (without the quotes). Whatever else one believes about Google and their ethical and legal standings, their search engine is far more likely to produce salient results than the built in search here on the Brickboard. Another trick there is to include "200" as a search term, limiting your results to the 200-series forum.

You will have to describe the knocking sound better. Under what circumstances does it occur? Is it constant? Is it speed dependent, or more likely to occur on rough roads?

The most common source of rear-end noises in my experience has been the torque rod bushings which wear out, causing clunks and knocking sounds especially on acceleration or stopping.

Most vibration/resonance issues in my time driving 240s have been the result of unbalanced tires/wheels. I always check there first. Since I get free rotations where I buy my tires, it's a no-brainer.

Good luck, and try to describe the conditions of your knocking sounds better.