Since the vibration here is most apparent at one particular
RPM, there might be a resonance at work, and resonances are often
weight-related; the weight of the engine, for instance, and then we
would suspect mounts, or maybe an exhaust pipe buzzing against the
cowling.
Could be something else. Pulsating vibration can be caused by the
interaction of two rotating components running at other than
whole-number ratios. In this case, it might be bad generator bearings,
since the gen is gear-driven and (I believe) doesn't run at any even
ratio with the crankshaft. Pulsating rumble in auto engines is
sometimes caused by worn main crank bearings; as the crank turns, it
"walks" around the bearing and causes that sort of noise. Metal in
filters or falling oil pressure are symptoms.

Dan