Rattling sounds when red lining.

Rattling sounds from engine when revving to red line.

So me and my friends just bought a old Saab 9000 for $250 and there are some problems with it, but the biggest problem is the rattling sounds that comes from the engine when red lining it. But sometimes when we are red lining it, there are no rattling sounds? What does that sound mean? I've watched your "how to fix rattling sounds" video, but because the sound only appears after 6k revs i don't know if it's the same problem. And the sound does not come from the belt, so i don't know if the crankshaft or anything is going bad.

don't red line that old clunker. Just drive it and pray it lasts a while. those are money pit machines and engine is obviously worn inside