engine knock 5.0 Crown Vic

Hello, I've got a puzzling one: My '91 Crown Vic 5.0 liter has a slight engine knock on deceleration from highway speeds. This knocking only happens when the engine is warm and has run for a good 20
minutes or so. It doesn't knock at slow speed (low RPM) deceleration. And it doesn't knock if I disconnect the SPOUT connector (when checking the timing). The timing is set to factory spec. and other than this, the engine runs very well. Any idea what could be causing the knocking? Thanks, Tom

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