Rattling Noise
 

I have a 2005 Mazda 3 with the 2.3 4 cylinder with a 5 speed with about 50K on it. The engine began making a rattling sound like it had marbles in it above 2000 rpm. No one I took it to could diagnose the problem. It got louder and one day while on the highway, the engine blew up, completely destroying it. I purchased another engine with 23K on it, had it installed, things were fine for a month or so, now it is beginning to do the same thing. Any ideas?