2001 F-250 V-10 Noise under load
 

I have started hearing what sounds like an exhaust leak, but no leak has been found. When pulling away from a dead stop, accelerating up a hill, or when a heavy load is on the truck, is when this noise is heard the most. I have done the following to try an isolate the issue. Replaced all spark plugs, replaced one coil and moved it to all 10 plugs, no difference. Had the codes checked several times, but each time it says I need to drive another 100+ miles. Any thoughts on what may be the cause of the noise, and the reason the codes can not be read?