Onan Microquiet 4000 making grinding noise, won't start.

I had that going on couple years ago. Starter had enough battery juice to "throw" starter thing to engage, but not enough to keep it engaged. And made treble noise as plastic starter teeth raked across gears coming back into its resting spot. But I didn't know that at the time.

After ton a of reading and thinking I was in big trouble,

I started with fully charged battery after cleaning up battery posts and connections and that solved my problem.

But you might have other stuff going on. Start with easy fixes first.