....and not while just coasting, even if you are at the same revs at which you can normally hear it (on an effected engine).

Slap is always more audible when the engine is under load i.e. accelerating, either when actually driving or bliping the throttle. Under these conditions the tiny sideways movement of the piston is more violent. The amount of ignition advance under these conditions also has a large effect. If the Scoob had a temp. controlled ignition advance damper, which held back much of the ignition advance until the engine reached full temp. many people would not realise there was a problem! (You dont need all that performance when the engine is cold do you? - If you do then you have no mechanical sympathy and are making the problem worse!) I assume the Scoob does not have such a device? There have been, and I assume still are, cars produced with such devices for that very purpose.
JohnD