I'm perplexed with a sporadic, strange mechanical rattle at startup. It happens maybe once in every 20 or so starts, sometimes the frequency is more common.

If you listen closely there is a "TAP, TAP, GRRRRR", that's rather loud and pronounced, and then at the 4 second mark the engine sound goes back to the normal smooth mezger pattern.

 

Judging on your description, I had the same. This was after the emissions recall work that was performed. After a week or so, it disappeared. I think it was something the exhaust shield rattling against something

 

Maybe chain tensioners?

 

I was thinking the same - however if it was part of the exhaust wouldn't that noise be there all the time, and progressively get louder on open throttle? It's only on startup and sporadic.

Another thing I'm thinking about is the alignment of the transmission - these cars have that loud blip on start in some cases - maybe that has something to do with it?

Have no idea how even to start to diagnose the problem.

 

I had similar noise, on cold start I heard a rattle for a couple of seconds, then it went away. At that time I was using Castrol EDGE Professional TWS 10W-60 oil, maybe it was too thick cold and tensioners did not handle that well. I switched to 5W-50 Mobil1 and no rattle anymore.

 

This has been mentioned before and I came across an article in Porsche Excellence magazine a while back that explained that the 2007 and 2008 Turbos, last of the Mezger engines and GT1 block, are known for the timing belt chain noise and how the engine gives off some terrible rattle noises. Bottom line: No worries, this is normal. If it is something way more serious, you will know about it by poor performance or warning lights, codes, etc. Don't sweat the small stuff

 

It sounds as if there is not enough oil in the top ( side ) end of the motor on start up. Almost like a valve clatter until it builds oil pressure. Does the noise disappear when the oil pressure goes up on the gauge?

That would concern me too. What oil are you running, and when did you check the level last. How close to a service being due are you?

Mike

 

Sounds like normal Mezger engine cold start "rattle" to me. My 03 Turbo makes a similar sound at cold start. The longer the car sits unused the worse -- to a point -- the noise is. The car gets used every other day or so most of the time but once in a while it can sit a few days unused and the noise is a bit more pronounced.

Some of the noise is just the lifters pumping up along with the chain tensioners developing some pressure coupled with the engagement of the low lift feature of the intake valves.

The oil pressure pick up is at some distance from the oil pump and watching my car's oil pressure gage as the engine is cranking I see oil pressure rise to nearly 1 bar then as soon as the engine fires it shoots up to 4+ bar in nearly a blink of an eye.

Be sure the oil is reasonably fresh. I change the oil in my Turbo every 5K miles. I run Mobil 1 0w-40 (the engine has almost 115K miles on it). In the summer I tried Mobil 1 5w-50 not because of any concern about the noise but simply because of some concern about the air temperature and the possibility the engine was running warmer than usual. The oil pressure hot wasn't any different and the start up noise was the same so I went back to Mobil 1 0w-40. (I run this oil in my Boxster with 275K miles on its engine and it works just fine.)

Anyhow, be sure the oil is reasonably fresh and the level is ok. I like to when the oil level drops enough -- 3 bars/segments -- to add a quart of oil to bring it back to if not at the max line/segment to the one directly under it.

Last but not least maybe you need to use the car more often?