I think people aren't used to hearing a larger quad cam motor with 4 chains driving em. That's a lot of mechanics moving around during startup that take a lot of oil. It shouldn't be as noisy when it warms up.
If it's anything like the ford DOHC V8s, its a fairly oil dependant setup and the sound will change if its warm.
That's why the motor revs high when it's cold, it's to get the oil flowing! I know my DOHC Cobra oil pressure would shoot way up to around 100PSI when cold to get everything oiled as quickly as possible. I'm willing to bet if you hook up a pressure gauge where the sending unit is for the factory sensor, you'll see a huge difference in pressures. The factory gauge is kind of a joke as it's calibrated and doesn't display actual pressure. They do this so people who don't understand what's going on won't flood the service department with "non-issues".
Listen to the different parts of the engine with a stethoscope if you're concerned; you'll find it to be the valve train. 2 large chains run from each head to the crank, and 2 smaller chains connect the secondary cams to the first. Variable valve timing is acheived by varying which side of the chain receives tension. I've rebuilt ford DOHC motors before that didn't have VVT, so I'm not sure how Toy. goes about it. That's the general concept though.
Here are images of Ford's 5.4 DOHC w/ roots blower
