What you describe sounds like square waves are coming from the voltage converter. I imagine it contains a switching type of converter, since you say it has two switch positions.
Have you used a converter which contains a simple 1:2 step-down transformer? It produces sine waves. It might be larger and more expensive, but quieter with your clippers. They are more in the category of a motorized appliance.
Whereas you're using a converter which is better at charging shavers or powering curling irons, etc. Appliances which tolerate square waves.
Question: Is the frequency in Iran 50 Hz (typical outside the USA)? While this isn't ordinarily a problem, however in your case being at a lower frequency could bring out unexpected resonant behavior in the clippers. Causing shaking. If the clippers contain a coil (inductor, choke) then it may be reaching saturation at the lower frequency. Causing buzzing.
I'm just brainstorming here.