What could cause a lout POP when starting a 66 chevy?

i have had the thing for almost 3 years now and it has run very well b/c the engine was rebuilt when i bought it with only about 2k miles. But... I have heard this really ugly sound about 4 times now over the last 3 months. Its very loud and sounds like a rock smacking against the underside of the hood. This last time i heard it it was the loudest. It happens the instant i turn the key. Instead of the engine reving up, it makes that pop... which shocks me and i turn the key back instantly to stop ignition. I haven't ever kept the key turned after the pop for fear of damaging something... always let go the instant i hear it. Then i turn key again and the thing starts up without the pop. what the hell could that pop be? i don't know details on the specs but it has one of those fancy msd starters that sparks several times insead of once... and its a 383 with a high rise and carb. What could it be and why is it happening now and only 4 times in 3 months when i drive it every day?
and its cammed out, or whatever you call it.
the pop is coming from the engine compartment. it sounds as if it is coming somewhere close to where they key turns, on the other side of that wall, on the driver's side.
There was soot there! Why is it doing that?
 
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