I have had more than a few frames with bits of filings or crap in the tubes but I did recently get a frame that really seemed to have a lot in the seat stays.

I did the same trick as paraWK4 and I slowly got it out. It looked like fine sand but I put a magnet to it and was shocked to find out it stuck. It is fine rust flakes!! The rest of the bike is fine so I sprayed Boeshield in the breather holes and try not to think about it.