2012 BMW X3: Wastegate Rattle
April 26, 2012
At least, that's what I think is causing our longterm 2012 BMW X3's cold start rattle. There's a muted buzzy rattling sound in the background that gradually fades away within fifteen seconds or so after a cold start.
To my ears this is textbook wastegate rattle. Cars that have vacuum-actuated or electronic wastegate actuators hold the 'gates open during a cold start in order to rapidly put some fire in the catalyst bricks, thereby reducing emissions. The open valve then dangles freely and makes the jingling noise until it expands and takes up the clearance. Our longterm Kia Optima Turbo did the same thing.
In general this issue is benign and falls into the 'annoyance' category, rather than the harbinger of an actual serious problem, but not always. We'll have it looked into at the next service.
Jason Kavanagh, Engineering Editor