So I've seen it happen where a decent vehicle would be driving along and all of the sudden (usually) while getting on an on ramp or a similar situation would start rattling. Beginning of a thrown rod. But what causes an engine that seems sound and makes no noises all of the sudden to rattle. Whay happens inside to cause this. I had it happen to me once. Shifted from 2nd at high rpm to 3rd and my cavalier started to rattle. What happens to make an engine out of no where start rattling? Just curious. Thanks!