This is the intake. I didn't know what it was either and first, I had a general idea that it was the intake but wasn't for sure. I don't know if this is bad for the truck or not, but my buddy and I unhooked the top of the airbox and I would hit the throttle for a split second and you could really hear it whistle. I am guessing this same noise you would hear all the time with an aftermarket cold air intake that isn't in a closed box. It definately is nothing to worry about though.
So basically its a cool way to pretend your truck has a lot more horsepower than it really does though i really wish i had a turbo...
I posted something about this two years ago. It really is not a whistle, more like a ringing sound, when you think about it.... on heavy acceleration it begins to ring, whistle if you like, around 3000 rpm.
I discovered this in the most offhand way. When I was screwing around with the belt squealing noise I pulled the serpentine belt off, and removed the idler pully from the tensioning arm. If you look at the idler pulley, it is held to the arm with a single bolt. However, the bolt goes through a stationary flange and then through the pulley in to the tensioning arm. When I pulled this apart the flange came off with the pulley and I dropped the flange on the garage floor and it "rang" at exactly the same tonal frequency you hear when the engine whistle/rings on hard acceleration. If you take the thing apart yourself and look at this flange, it is shaped very much like a bell. Just for yucks I reassembled without the flange but just used a thick washer in its place and the sound went a way. I don't know what the purpose is of the flange and I don't find the sound that objectionable.... now that I know what it is..... so I put the flange back on.
I have attached a pic of the pulley and flange- I placed a red paint dot on each one so you can see what I'm talking about.