[henry L.]

i have a 2004 f-350 superduty powerstroke. lately when stopped and idling the normal engine clatter stops and the engine can barely be heard. there is no drop in rpm's and the engine doesnt die. when i touch the gas it comes back to normal. when left alone it is only quiet for about 15 seconds then back to normal. any ideas? is it something i need to have checked out? thanks