heliman500,
The natural per-blade vibrations come from the rotor as each blade creates a part of the total lift the aircraft needs. The number of blades determine the principal frequency of the dominant vibration. We speak of the vibration as "N per revolution" where N is the number of blades. Think of this n per rev as like the 60 cycle hum that audio engineers spend their essence removing from the music we listen to.
Where does it come from? The rotor is really a nervous hum of resonances, the dominant resonance is the 1 per rev flapping oscillation that each blade experiences. For a 4 bladed system, the sum of these 4 individual 1 per revs is a 4 per rev.
Most rotors spin so that the blade tips are going about 725 feet per second, which means that a 20 foot radius rotor spins at 5 turns per second, and so 1 per rev is about 5 cycles per second (5 Hz). A 15 foot rotor spins about 33% faster, so the 1 per rev is about 6.5 Hz.
A 4 bladed 15 foot rotor has a 4 per rev of about 26 Hz, if it had 2 blades it would have a 2 per rev of about 13 Hz
A 20 foot rotor has 4 per rev of about 20 Hz, with 2 blades, it would have a 2 per rev of 10 Hz.
Generally, the fewer the number of blades, the higher the amplitude of the N per rev, so 2 blades have a higher n per rev than 3, and so on.