Have you ever dropped something down through the engine compartment and can't seem to find it.  Here is a possibility. Just helped a friend with a Rotax 912 UL who reported a curious high pitched grinding at the aft of the engine while moving the prop.  Turns out it was fragments of a wing type camloc and a whole camloc which he had dropped at some point in the past which had just recently found their way between the coils of the stator and the inner magnets of the flywheel.  The bits were scraping the inner aspect of the flywheel as the magnets attracted them and the stator fingers kept them in one place while the flywheel moved by. His engine had no cover over the flywheel as some engines don't. Seems there was no harm done, just some grooves in the flywheel. Glad he noticed the sound on preflight.