2004 concorde 3.5 engine knock

My 75 year old dad has a 2004 concorde 3.5 that has developed a knock. Dad and i both agree that if it was the old days the knock would be a rod. He Has had it to the dealer several times and they hear it but cant find anything. the car has 38,000 miles. The knock starts at arount 1000 RPM. anyone have any thoughts on this? could it be something other then a rod? Thanks for any ideas. we are planning on being a pest at the dealers.