The higher cylinder temperature when cruising at higher speeds allows for a more robust combustion event (as does higher cetane fuel).

Once the cylnder temperature comes down, like sitting overnight, the knock returns.

Blipping the throttle introduces more air incursion (think venturi effect).
This dampens the injection event and recreates the knock, even when the cylinder has not yet cooled.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022