I've got an 04, 6.0L 350 with a cute whistle from the coolant cap. Seems that it only hapens above about 2000rpms, sometimes a little higher, but lower ( still above 2000 ) with higher ambient atemps. The temp gauge never goes up. Alittle coolant does come out of the tank if it's close to full. Doesn't seem to matter how hard it's working,. The noise starts imediately with the rpm increase and ends abruptly when the rpms are reduced below 2000! The only things I come up with are bad, could I be missing something?

 

Sounds like a possible cracked EGR cooler.. It can be testedin place by pulling and holding a vacuum on the cooling system.. Sometimes you'll see coolant/carbon sludge under the EGR valve as well.. Could possibly be headgasket/s, but generallly wouldpuke more coolant out of the degasover an EGR pressure.. Failing gaskets may make for a faster coolant drop..