1968 Buick LeLabre: Engine Makes Rattling Sound Under Acceleration HELP PLEASE!!?

could be a drive belt slightly loose that it is hitting one part when at a certain rpm. Try going as fast as it takes to make the noise in 1st gear and do the same thing in reverse and see if it makes that noise. Also try revving it in neutral to see if it appears there(no higher than 4000 rpm though when doing this; it will kill the engine)