Need input/ideas 2000 5.3L V8 GMC Sierra Starter???

My girlfriends 2000 GMC Sierra has got some issues. I tried to crank it the other day and it turned over but then made a clank clank noise and then a high pitched whine. Sounded like the starter engaged then immediately started free spinning like it wasn't catching the flywheel.
So, I took the starter off and took it to Advance Auto to get tested.
Well, turns out the starter is fine. It is engaging like it should and no teeth are missing off of the gear.
When I took the starter off, I looked at the flywheel (didn't try to turn it) and it looked to be fine.
I'm at a loss now, I don't know what it could be. Thought for sure it was the starter.
Aside from the fact she needs to get a Tacoma or an FJ haha, any ideas or input on what might be going on would be greatly appreciated.
Thanks all!

 

Did you actually watch the starter as it was tested? The reason I ask is because I went with someone to autozone to get a new starter and when they tested it, the machine said that it was good, but the gear on the end was not spinning. We already knew the starter was bad but they checked it anyway. If we wouldn't have been standing there, they might have told us it was good and to try something else.

 

Yea, I didn't the first time, but I ask if he could test it again. He did and we both leaned over and observed it engaging, spinning, and then disengage. I'm hoping its not the timing belt/chain broken or anything like that. I just don't know where to look next.

 

If the starter was really catching the flywheel, then the engine would be turning over because the flywheel is connected to the crank. It sounds just like what my s10 started doing a while back and we changed the starter and everything has been fine. The clanking sounds like it tries to catch the flywheel then the starter lets go and starts spinning free.

 

Ok. So even though the starter tested ok, it could still be malfunctioning when a load is put on it? I'm wondering if I bought a new one, and it didn't fix the issue, if I could take it back. Gonna call and find out.
Thanks for your advice!

 

No problem. Yea that's what I would try to do.