Unidentified Slapping Sound Coming from the Motor.

Have a '95 6.5L that has been making a slapping sound for about to months. took it to the mechanic twice. first he couldn't find anything thing and said that everything was good. The next time I took it in he located the noise in cylinder #1. He switched injector and the noise didnt go away but when he shut the cylinder off it did. he thought it was the pre-cup so I pulled the head and found that wasnt the problem. checked the valves to see if they were wore and they were fine and checked the lifters and they were fine also. Last winter the injection pimp was rebuilt. Do i need to dig into the block or just tear it a part and rebuild? trying to get it up and running before harvest.

Welcome aboard!

Before you dig any deeper, check the crank damper and pulley. They often make "slapping" sounds when they fail (both are rubber isolated). If they're original, they should be replaced, in any case. A worst case scenario would be a broken crank (possibly caused by a failed damper).

Piston slap? Is it more or less noticeable under heavy vs. light (idle) load?

As Maverick mentioned....the rubber dampers on the belt pulley and damper.

I have not heard of #1 piston being shut down stopping the damper/pulley noise.

A bad wrist pin bushing comes to mind though.

A failed wrist pin bushing will allow the piston to hit the head.

If the head is still off try this.

Rotate the engine by hand to get the #1 piston moving down from TDC...
After the piston is down a 1/2 inch in the bore.... use a hammer handle and tap the piston top with a good thunk and see if the piston moves downward.

Can you get a good close up picture of the piston top and post it here please

He said the noise went away when he shut the cylinder off. This rules out the damper or pulley. I would be looking for a burned or galled piston. Damage should have been visible with the head off, though.