I have similar noise on mine and the tech said I should try using higher octane gas...I only use 87 octane. Going to try this when tank is near empty. He also suggested the Nissan oil filter would help and the exhaust manifold was not cracked or the culprit. He didn't think the valves made this noise because it only makes the noise when you accelerate.

We will see what happens with better gas and oil filter, but I don't remember this being a problem when I bought the truck Oct. 1999. The engine got the Ever Wear oil treatment (similar to Slick 50) at around 30k miles and this noise started back in Feb. 2003. Currently 68k.

Could be nothing!