Loose main bearings can make a deep clunking sound, and certainly show up as reduced oil pressure, but in my experience, they won't throw "shiny metal pieces" which would be trapped in a filter.
The sloughed off tin/babbitt coating would be dull and probably not magnetic. Is it too late to take a mechanics stethoscope and find out where this sound is coming from. It could be much less catastrophic than you think it is.
Stuart
