What you are hearing is the cams being advance to reduce emissions in cold start, both cams will advance 35 degrees for about roughly 10 seconds and that what causes the rough sound like a exhaust leak .

Hope this helps

Sam

. Thanks, Sam. Exactly what I wanted to know

90% of HC emissions is at startup. Cold engine = poor atomization of the fuel which goes right out the tailpipe. The VVEL does its thing to prevent unburnt HC's from going out the tailpipe and warm the engine and cat convertor quickly to reduce HC output.

"Cleaner emissions

Intake-valve timing is optimized on startup, when the engine is still cool, to quickly raise the temperature of exhaust gases and more quickly activate the catalytic converter.

Hydro-carbon emissions are reduced in the low-to-medium range by keeping intake-valve lift low, speeding intake flow and dispersing the fuel into a finer mist, resulting in more efficient full combustion."