Pull the starter and look at the starter gear. See how far forward
the starter gear is shimmed. If you have another starter lying around compare both to see if there is any differences on how far forward the starter gear is. Could be the starter gear rubbing on the flywheel.
This happened to me a long time ago. Installed a rebuilt starter and I was getting that same rattling sound when driving. Turned out that the starter gear was shimmed forward too much causing the starter gear to rub on the flywheel.