1999 Ford F350 Engine Makes ticking noise when under load

I work for the U.S Navy, and I am trying to determine what a ticking sound is coming from the engine of a Ford F350 Dulley. It does not make this noise while at idle, but when put into drive and driven the sound becomes noticable from inside the cab. I thought at first it was an exhaust leak, but it does not make the sound when at idle and I am under the hood. I have tightened all exhaust connetions to and from the turbo, manifolds etc. Again the engine only makes the sound when under load.