I bought a 2000 2500 5.9l with auto, I have put 6,000 miles on it and it now has 253,000.
Don't know a lot about the service history of it, but I believe its the torque converter. It sounds almost like a power steering pump going out, it disappears only when I put it in neutral. No noise when in park and it can only really be heard when it goes into 2nd and up.
The transmission was flushed at 225,000. So I was told

 

Guess this is uncommon, or I must be hearing something else

 

I sure can't say it is common, sorry I can't be of much more help.

 

Thanks anyways.