Does this happen all the time? If so, it might just be the turbo. I don't know if they've made updates to them on the newer models or not. By the turbo I mean, on the 6.6 Duramax in the full-size trucks, from 2004.5 and on, they have used variable vane or variable geometry turbos (2001 to 2004 used fixed vane turbos), so at idle, they would switch from open to close on the vanes depending what the engine needed, thus giving you a different exhaust tone. Our truck use a variable geometry turbo I believe, so there is the possibility that they do the same thing.
There is also the possibility that it could be something to do with the regen process as well, if you are doing a lot of short commutes and not getting up to speed for long enough for the truck to regen normally.