Vibration Under Front On Bed

2005 Tacoma
I4 Reg Cab 4x4
3" lift 265/75/16
Magnaflo muffler 2 1/2" pipe
K&N CAI

Well recently, since summer has started and I now leave my rear sliding window open, I have been hearing a vibration that seems to emit from up between the cab and box of the truck. I hear it when im doin anywhere from 45 mph up, and I only hear it when I let off the gas, or put it in neutral and coast!

Another sound I hear coming from the same area is almost a metal like rattling sound that I get when accelerating, I only hear it when around 3000-3500 rpm (which I dont have to hit to much) and If I was to keep a constant speed at 3000-3500 rpm the sound does not last more then a second or two! I have time to get a closer look under the truck for the next few days to see whats up, and help would be appreciated!