Not magnetic would mean thats its probably aluminium. The crankcases are aluminium i think and also the pistons. I cant think what could cause the swarf though unless its manufacturing debris or a bearing somewhere has seized and it turning in the case.
The ST2 engines are pretty bomb proof compared to the four valve motors so if its quiet when its running i dont think i would worry about it.
I suppose they could also be pieces of white metal or phosphor bronze bearing shells?
Anyone got any more ideas?