I had this very problem. First with a couple of Droids, then with my first two iPhones. Got a third iPhone within the first year of switching over because of this issue.
Yes, it is metal dust getting into the speaker where you listen. Speaker=magnet.
I got a Lifeproof case (found them cheaper...
Been making plans to expand my usable square footage and build a shop in the back yard. Lots of reading and collecting of knowledge. Lots of really cool stuff and creative folks keeping the inspiration flowing.
Thanks
Jonathan