Clicking sound when under a load

When you start up a incline you will hear a loud metal clicking sound. Let off the gas and back on the noise quits. About a year ago I put a new engine block in the truck transferring intake, alternator, starter and the outside stuff. Must be something I put on because it did it with the old engine and now does it with the new one. I thought it was the clutch fan so I put a new one on it. Still makes the clicking sound. Could have gotten a bad new clutch fan? But kind of doubt it. Your help would be greatly appreciated.