Warming up the engine
During the warm-up phase, the high-performance V8 engine runs a little more roughly due to its emissions control system.
For technical reasons, the exhaust systems sounds slightly metallic when the engine is cold.
This information can be found in your manual on page 121
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2008 E93 335i Black Sapphire Metallic / Saddle Brown
1997 840Ci Jet Black / Black
2004 E46 ///M3 Laguna Seca Blue / Black