hmmmm. do you have stock exhaust? or? more specificaly the down pipe. i had a rather large DP squeezed in my 96 and at certain RPM's it would come across as a knocking/ grinding sound.

Ford dealer told me it could be a injector not firing, that what it sounds like, does that sound right

it very well could be the injectors, you need to run a buzz test to determine what injector(s) are bad.

What is a buzz test ? Were do I go to do this

A buz test is done with a scanner that works on Powerstrokes.
It sends a signal to each injector and waits for a response with engine off. This can tell if the electrical system side of the injector including the IDM,harness,uvc harness,injector solenoid is all working. The buz test test each cylinder.
The buz test does not check the mechanical part of injectors such as poppet valves or nozzles. You can pass all buzz test and have bad injector.
Cylinder contribution test using same scanner will test with engine running and show if any are not contributing power. If its passes the buzz test i would pull the valve covers off and start the truck and make sure all the injectors are pumping oil. A good scanner is the AutoEnginuity.

Ok I'll try this and see if it's the problem thanks for all the help, much appreciated

good luck:c: