Horrible Ignition noise when warmed engine

Hello. I have a 74 Duster with a 318, Edlebrock 4 bbl, auto., mild cam. A few weeks ago I took her to a birthday parade. Drove about 45 mins., turned her off for about 15 mins. When I went to start up, there was this horrible tatatatatatata noise, and wouldn't start. Someone jumped me and it drove home fine. Started up next day fine. Yesterday took her for a drive, stopped for gas, turned her off for a few mins. and same problem, tatatatatatata noise. Someone jumped me again. Had battery checked out and its fine. It does have electronic ignition. Any thoughts??? Thank you for any help.