It turned out to be a bad ignition switch. Turning the key back to the normal position would not always cause the starter to disengage. Not sure why shifting out of park and into drive would do the trick. Thanks everyone for your input.
The reason the noise ceased when you went to Drive is that the neutral safety switch would open in Drive shutting the starter motor ‘off’.
Good for you, @mschuster. You may be in the market for a new starter soon, though.