No rangehood can be truly silent because the fan motor and moving air necessarily produce noise. If the fan motor is in the rangehood itself the vibration as it operates will be amplified by the rangehood body and the moving fan blades create air turbulence, which is also noisy.
If the fan motor is removed from the rangehood itself and placed at the other end of the ducting in another part of the house then it is likely to be quieter. However the motor and fan will still be producing noise and at 720mph it travels faster than the air through the duct so can (and does) make its way back through to the rangehood. Obviously every rangehood installation is different so results will vary from case to case.
You might be tempted to spend a very great deal of money on a 'silent' rangehood but in this writer's long experience with selling rangehoods and ducting a really truly is not possible. And even a quiet rangehood can be made noisy buy a poor ducting installation. Unless you have money to burn, probably the best plan is to buy the quietest rangehood you can afford after auditioning a few and then investing in good ducting, making sure that the best duct route for your house is used, i.e. roof, wall or soffit. Remember that a shallow roofline may not have enough space for the duct to pass through without first being squashed, which will affect airflow amd noise.
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