I also notice the issue on my 2010 Ram Big Horn (5.7L) with 86,500 miles on it... only happens at low RPMs, but can't get it to do it consistently (first noticed while going through a drive-thru). Brought it into the dealership to get that and some other things looked at, but the dealer thought I was crazy until I pointed out exactly what I meant. My friend who works at the dealership didn't seem overly concerned (no surprise there), but also thought it was something with either the exhaust vibrating or some kind of heat shield. I'm hoping to get it into the shop next weekend... I'll hopefully be able to update everyone then.

 

Easy fix. I put a two chamber flow master muffler on and now the truck sounds awesome and the noise went away. Let the hemi roar