Valve noise?

I've noticed for a while that when I put a load on the engine, the Taco sounds very much like my old 76 Nova used to when I floored it. That is to say, there's valve noise. At least as far as I can tell.

I can hear it clearly everytime I'm going up a nearby hill (the trans has already hit top gear) and I give her a little gas. If I stomp it, you hear it, but not as much since the engine kicks down.

Is it just because I've been driving a 4 cylinder for so long I forgot what a 6 sounds like when it's got a little stress on it or have others noticed this?

 

Thanks. I'll try to see what happens but if they bumped the octane rating for '08, they forgot to update the owners manual and tell the sales team at my dealership.