I am on a Juki FB page and have seen this mentioned several times. It's a problem with the walking foot rubbing metal to metal and it's tiny metal shavings you are seeing. I have the TL2010q and haven't had the issue, but I think you aren't alone. A new walking foot might help (I've heard people mention a Janome one instead?) but google the issue (Juki walking foot metal shavings) and you will likely find some advice. Hope this helps.