I had my '13 in the shop today for something else, had them check this noise. The explanation was that it is a secondary air pump/system that kicks in at startup for about 30 seconds for emissions reasons. So, it sounds like I got the same story everyone else gets. It sure isn't a comforting sound though!!!

 

My 2010 did it...so does my 2013. No big deal......but obviously its an issue for some people. I specifically recall when I leased my 2010 that the sales guy had me wait in the showroom while he started the car and pulled it around front for the test drive. The funny thing...the car was already within one or two slots from the front door and it was nice outside. Maybe he was just being polite........probably NOT.

 

I think it's because the oil is sitting in the oil pan at cold start and takes a few seconds to flow throughout the motor. Nothing more than that.

 

Typical characteristic of the G37 engine. The jury is still out on whether this has any adverse effects on longevity and durability of the engine. It seems like not much problems reported on this engine in its 5+ years of deployment.

 

Had a M35 that did that when I got back from vacation. Not going to lie it scared me, but it was normal lol

 

Yeah, the harsh engine noise is normal.