It is the black pastic piece on the top of the motor...says 6.6L turbo diesel.
I removed mine after inspecting it for cracks and found one along the seam...in theory, unfitered air can get into the engine through it.
I used one of the 2" expandable pipe plugs (kind you tighten with a wing nut) and drilled a hole through the bolt and ran a cotter key through it. This way if it would come loose the bolt cannot fall into the intake and be mulched by the charger.
I noticed very little difference, turbo whistle is slightly louder but probably not noticable to the untrained ear.