Per Pete Jackson ...

Fiction mixed with fact : Gear drives create harmonics

Fact : Every rotating part that contacts another, or part that is stressed under load in an engine ( crank, cam, valve train, gears, chains etc. ) creates a harmonic ( frequency ) . Some frequency are harmless, a simple matter of unavoidable friction , while other are percussive & concussive, and are capable of creating damage in very short periods of time. Gear drives, chains ( yes chains ) and most valve train components create frequencies which are a simple matter of friction and rendered harmless by simple lubricants (oil). An unbalanced or improperly balanced crank , ( especially if the harmonic balancer is removed ) create the percussive and/or concussive types which can over a period of time ( if severe very short ) cause damage , above and beyond anything considered normal wear . Gear drives absolutely do not create these types of harmonics .