Sounds like the infamous HEMI tick. I just did my first oil change and now when I first start up I hear it. Once it's all warmed up I don't hear it anymore. From the reading I have done it's not a problem, check out this thread on the subject, http://www.ramforumz.com/showthread....ight=hemi+tick
If your concerned you should get a second opinion from a good mechanic.