I doubt that filters on the servo leads will help you. Your receiver has bypass capacitors in it to remove any pick up in the leads.
Here is another test you can make:
Remove the spark plug from the engine but leave it connected to the plug lead. Wrap the spark plug with aluminum foil completely enclosing it.
Shut off the gas from the engine, turn on the ignition, and with the spark plug hole open spin the engine up to speed with a starter. It might be a good idea to remove the propeller,too.
If the problem goes away you have a vibration problem.