That certainly sounds like the noise I was getting. Mine started out a light knock that would go away once the car was warmed up. It quickly got worse. It was loose bolts on the VANOS intake side sprocket. It means the bolts have worked themselves about half-way out. Mine has a build date of March 2006, so very close to yours. It's a little early for the valve adjustment, but while you are doing the VANOS, the valve cover will already be off.
Beisan Systems offers some great kits. I feel like my VANOS is now rock-solid.
Here is the sound I was getting:
Salty