Wouldn't you hear any recip noise (like a wrist pin, or loose flywheel) at any rpm, w/o putting the engine in a load situation? Those things you're talking about could be heard in park, maybe only needing some throttle jabs.
Getting it in Neutral as well . But you really have to stab the throttle to get it to occur vs driving and lugging the engine down . My other thought and really my first thought was that maybe there is a lifter bleeding down under load .


