npalao, S8RAH, red_S - just to let you know, my car also produces a similar knocking noise as you describe, but as it is only ever apparent at low speeds over certain kinds of bumpy surfaces (and with the stereo off) I've lived with it. It's funny that when you read about someone else with it too, you suddenly become more sensitive to it! I've now had my head under the bonnet trying to wiggle gear rods, etc
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S8RAH, have you made your dealer aware of red_S's suspected cause? I think he may be closer to identifying the culprit.