Everytime I read posts about the 6.0 exhaust turbo whistle it makes me miss my 2006. I had a MBRP 4" turbo back exhaust with CAT delete. That thing whislted like a 747 preparing for take off... and once you stomped on it sounded like a big block from hell! In my opinion, its the best sounding exhaust noise of any car/truck I have ever heard. I have tried with the 6.4 to replicate the sound... but it just doesn't come close. I have a AFE intake, 4" ceramic down pipe, stock exhaust with cat/dpf delete. It makes some noise, but no real whistle. When I stomp on it that big block noise comes out a little bit.