>Aluminum, because none have attached themselves to
> my magnetic drain plug.
> 
> Now, wouldn't that exclude rods or bearings from being suspect?

uhhh, unfortunately no.  The rod and main bearings are a nonferrous composite 
material made of all sorts of secret things but contain Al, Cu, Ni, 
Pb, etc.  Yes the rods themselves are a ferrous (magnetic) metal but 
unless you have extreme damage, you won't be seeing parts of them 
(yet).     Looks like an engine teardown is in order if that nasty 
noise is still there.  Maybe someone else has better news (ideas).

We're keeping our fingers crossed for you...

-dan