Mine has the same noise also! I adjusted the valve lash thinking that was it, but it didnt help, and I took it to the dealer last week to see if they knew what it was and they couldn't figure it out..I hear mine most when im cruizing in 4th (5spd) at about 1600 rpms but I also hear it at idle. An older 3500 parked next to me at Sonic the other day and I could hear it in his truck also...maybe its just a cummins thing?

 

Check the pulleys the other day since it sounds like a dry bearing. I checked all the pulleys and the power steering one wiggled a bit when running. Shut it down and loosend the belt and the shaft moved in and out about 1/4 to 3/8" play..... My quess is it shouldn't move that much, so maybe it's a power steering thing?? My truck never made this sound and then all of a sudden it's been getting louder and louder. Tone ring has nothing to do with it, this or that. Quad box makes it louder only because I'm sure it changes engine speed ect. So I'm waiting to hear from the dealer about what the problem is. Here's hoping DC can fix it