Does this thing have rear A/C? if so the rear expansion valve has been known to go bad and causing to much freon going back to compressor and cause knock.
Could also be a little low on freon causing not enough flow at low rpm, and not enough air flow thru condensor to make things work correctly.
So engine speed and vehicle speed come into play.
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64 Lemans hardtop
4spd, buckets
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