It could very well be the crank pulley loose on the harmonic. It happened to me in my parents' Caprice wagon with a 454 when I was a kid. Heard a rattle now and then, and you know I was always taking that sucker to redline every other minute.
Then one day I sent that crank pulley right up into the fan and radiator. Watch out.....check it out before you keep driving it.
