grinding sound

I am posting this on behalf of my father. He has a 2000 F-150, V8, auto, 4x4. Every now and then there is a grinding sound when he starts the truck. Most times it starts up no problem. Any idea what could be causing this?

you might remove the starter and take a good look at the starter teeth and more importantly, the flywheel teeth. One or two bad teeth make a lot of noise when they mesh just right.