This is the exact problem I came on here to post about.
Mine does this too. My truck only has 84k on it. I use to drive it 30 miles each way to work when I lived in Northern VA and never heard this sound. Now I drive it about 2 miles each way to work. The car doesn't get fully warmed up to and from work. The mechanic I took it too ( of course it doesn't make the sound when you take it to them) said since the car wasn't doing it when it was good and warmed up that he thought it was the lifters. His suggestion was to change the spark plugs (I changed the easy side to get to but didn't change the other yet), wires, use a good oil system flush (amsoil) and use a synthetic oil. He said he thinks that by not gettting it warmed up everyday that the hydraulic lifters are getting gummed up and thats what is causing the sound.
It really does sound like a diesel when you start it and it does diesel sound if you blip the gas real quick too. Almost like a predetonation sound.
The oil guage takes a few seconds to come up to full pressure as well. I am also getting a whislting sound that I haven't looked into now.
The longer the car sits the worse the dieseling sound is. It doesn't seem to affect the way the truck runs but it sounds like something is wrong.