2004 f-350 6.0 125,000 miles,
Do glow plugs act like spark plugs when they are getting bad. I have a random sound like spark knock when i am on the highway. I have deleted EGR and rebuilt oil cooler. Turbo sounds good. Use diesel power service summer additive every tank.

 

No. Nothing like it. They are used to pre-heat the cylinders to fire the motor when it is cold. Once the truck is running they really serve no purpose (on these motors anyways). My guess is injector going bad possibly... When's the last time the oil was changed and/or the fuel filters?

 

Have you checked for diagnostic codes?

I would check to see if your FICM is securely mounted (located on left valve cover) and that the rubber vibration insulators are intact on the 4 FICM mount locations and the 2 valve cover mounting studs.
Then I would check the injector wire harness for worn insulation or a loose or missing injector connector clip.
You may have an intermittent open injector circuit due to a loose connector or you may have an injector that is grounding out from time to time due to worn insulation.