2011 Ram 5.7 hemi has an engine rattle at highway speeds

I bought a '11 Ram with the 5.7 Hemi a month ago with 52k on it. It doesn't idle
smooth, and even when the rpms are up at cruising speed the rpm gauge jumps up
and down by about 100 rpm. What worries me even worse is at cruising speeds when
the truck is under a small load, ie going up a slight hill and not in MDS, there is a
pretty loud rattling noise coming from the engine. It's intermittent, but can be
replicated fairly easily. It does it right around 1500 rpm at cruising speeds in 5th gear,
not in MDS. Any suggestions? I took it to the dealer and the service tech took a ride
with me, he said he had no idea what it was. He said he would fill out some sort of
report to send to the engineers. I'm not sure if he was b.s.ing me or not. That's been 2
weeks ago and I haven't heard anything from them since.