Could be a loose timing chain slapping the inside of the timing chain cover. Also might be a bad lifter or just a need for adjustment. A header leak also sounds to me just like a clicking lifter.
David's right though, likely not piston/valve interference unless you've got new parts in there.
Rev
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'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.
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