Had a starter problem with my e34 540 the other day. Bought a new starter and flexplate, and then it turned out that the actual starter was loose, and was slipping when it engaged.
Didn't need the starter or the flexplate.
Now the noise that I got was different from yours, but it will cost you nothing to first make sure that the starter is securely fastened.
Failing that you will have to remove the starter to inspect it.
Now a lot of people say that a worn starter can destroy the flexplate, so I would recommend you attend to this ASAP. Sommer die naweek.
To replace the flexplate is a bitch, cause the gearbox needs to come out.