re:  droning at 3900 rpm

This sounds like the characteristic vibration that's inherent in all
inline four cylinder engines except those with balancer shafts.  It
manifests itself as a droning at a certain rpm range, which can be
tuned up or down the range to some extent but not out.  Should be
particularly noticeable following experience with a nicely balanced
(inherent in the design) boxer 4.  Since my Opel experiences of the
early '70s I've managed to maintain my oath to never own another
inline four.  Seems if going to the expense of a balancer shaft why
not the expense of more cylinders instead?