On old vehicles that can be caused by too much vacuum advance. With closed
throttle and vehicle keeping motor speed up, the vacuum advance working too
much, not every cylinder full of thinly stretched fuel/air mixture will
ignite well. Some of them with slow flame fronts will still be burning when
the exhaust valve opens and that makes exhaust intermittently pop.
On old ones, sometimes vacuum advance adds an extra 30 degrees from factory.
Better performance is usually obtained with somewhere between 10-15 degrees
of vacuum advance (in addition to centripetal.)
Mike
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