Sometimes when I start my 07 Si I hear like an engine ratle mechanical noise when it first turns over, only last for a split second. It doesn't do it all the time.

 

I have the same concern. i believe its normal.https://honda-tech.com/forums/honda-civic-2006-2015-79/08-si-valves-2404355/

And does it sound like this in this video on this guys first start up, the little rattle is what i hear right after cranking listen closely...
http://www.youtube.com/watch?v=yVJsnkcsvqI

 

I've only heard mine when I was inside the car, but it does sound like that video, just that first noise right after cranking that only last a sec then goes away.

 

Iv got that same noise goin on with myne. anyone know what it could be?

 

I think its the idler pully, ever since i changed mine out i dont hear the noise anymore..

 

Sounds kind of dumb but I was reading in the manual when I first bought my '08 Si and it said the noise is actually normal upon start up, as long as it doesn't continue. Can't remember what the manual said it was.