My son tells me today that when he shuts his car off he "sometimes" hears a "thump" sound. Said like an acorn hitting the car hard. Said about 10 seconds after shutting off and removing key. I tried to replicate it today but no luck. Any thoughts? 2006 x5 3.0L gas. 94,000 miles.

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2006 BMW x5 e53, M54, steel grey, black leather, pano roof, cold weather package, avic-z150bh head unit with JL Audio amp and speakers.