i have an 06 fx4. When i put the truck in drive and lightly press on the gas, just enough to add some pressure i get this shutter sound coming. it won't do it in reverse but i notice is also if i'm just idling through town and lightly step on the gas while coasting as well. at first i thought it was the timing advanced as i had put in a performance tune or my injectors making the shutter sound as the truck would be getting more fuel with the tune in it. I tried it on the stock setting and it is still making the noise. Anyone have an idea what that could be?