I indeed this will be interesting. I have a couple with the same problem, nothing but the tone is not red. I can ask 100 dealers in the same room with the coin in hand and get a different grade every time. It still is a beauty. Most will say MS-62. even with no marks at all on it.
They have a telephone number I can call during business days 8:00 - 5:00 I'll call tomorrow and talk to them about it. They also have an article on clean coins it tells some things but not what the reason for the call is.