Free samples that are quality are tough to come by. The Natural Studio free kit is good and has dynamic sampling (for realistic drum-replacement applications), although the samples aren't what I'd define as "metal." Well, Scandinavian metal, perhaps. You could always take the raw materials and effect/affect some attitude into them.
Beyond that, here's a few kick and snare samples. A few are free that I've gotten over the past years and others are from my private collection. Let me know if you need help with MegaU/L.