The most likely source for a rattle that I know of is usually the water pump. When this happens, if you grasp the fan blades with the engine stopped, you can usually wiggle it on it's axis indicating worn bearings in the pump.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!