This is a really old post so I'm hoping someone somewhere has some info. This same thing happened to me. I don't believe it is oil from the machine as it only happens when using the walking foot. My machine (TL2010q) is less than a month old and is doing the same thing. I'm sure it's been doing it from the first use but I didn't notice until I was SITD on white fabric. Most of it came out in the wash but some did not. If I take the foot off the machine and cycle the lever that operates the upper feed dogs, you can see it drop a fine dust which smears on a light surface. Does anyone have any ideas?