The fan is the component that sucks the air up past the filter and into the vent. If you find that the venting is not doing its job very well or the fan is making a distinct humming noise, you may find that grease and grime have migrated up into the fan and are starting to clog the spinning of the fan, and you need to clean up the mess. You can use a sponge or bristle brush to clean the fan with soapy water or a degreaser spray.
By cleaning the fan, you can increase the lifespan of the motor. Once the fan is cleaned and still experiencing problems with your fan speed, noise, or venting power, you may need to replace the motor of your range hood. Make sure to read the guide for the components of your range hood and get the correct fitting motor that is the right power for your range hood.