Does the engine knock at idle as well as when you accelerate hard? If it knocks only when you accelerate, there is probably too much advance in the timing and/or you need to run a higher octane gas (this problem will get worse with "winter mix" gases that have additives that make the gas more volatile for easier starting in cold weather).
When I purchased my Cutlass, the 455 had a knock at idle which turned out to be a spun bearing. Hope this helps.
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