Fluttering Sound from Engine, Only on Acceleration, Check Engine Light Did Not Light on 2006 Toyota Tundra

I have a fluttering sound, only on acceleration, that recently developed on my 4 1/2 year old 2006 Toyota Tundra. Gas mileage and performance have not noticeably changed. Truck has 125k miles on it. I have kept oil changed every 5k miles and performed most of the recommended service, including timing belt change at 90k. Suggestions were that this might be a warped exhaust manifold, but that has been ruled out. Someone else suggested that there is some sort of secondary air system pump and air check valves that could be the problem. I'm haven't heard of this before. Any thoughts..?

Asked by for the 2006 Toyota Tundra