2006 2500HD noise

This truck has a 6.0 engine with 56000 miles. When idling it makes a metallic tapping sound almost like a bolt hitting a shield. Sometimes the noise stops and other times it gets louder. I just recently serviced it and the noise is still there. If I rev it up just a bit it goes away. I have checked and wiggled every shield under the truck and it seems to be coming from the bell housing area. What have I missed?
Thanks
Mike