I was adjusting my new lights the other day with the truck idling and noticed a sort of hiccup sound. I pulled the top off of the intake its seems so faint but it echoes through the intake tube. It will do it every 30-45 secs or so and I just came back from the store so the engine was warm. Truck runs fine, haven't had any issues, 25,000 km.

 

I think I know the sound you mean... my '07 does it but I have no idea why.