What distributor are you using and was it modified at all? How long has the engine been running in this configuration? Does it handle more timing under acceleration with the vacuum advance disconnected? A fresh, clean Magnum engine with no carbon buildup and verified timing marks should not have any real tendency to rattle like this, in my experience.
Stock distributor using a FBO timing plate limiter. Left the stock springs. Can't have 2K on the engine since rebuild. Been trying with vac advance disconnected before taking the distributor out.
