The most common cause of this issue is shipping tape being present on top of the damper flaps, preventing them from opening properly.
To check that the damper flaps are working properly, follow the steps below: Unplug your unit and remove the ductwork from the top of the hood. Pull on the damper flaps with your hands to make sure they are not stuck in place, and that any tape or other debris is removed from their path. Remove the damper flaps one at a time and attempt to spin the fan with your hand, or using a long tool such as a screwdriver. If the issue persists, please look for any other blockage in the duct or any external dampers that are not opening properly. Feel for any abnormal vibration while in use. This can be a sign of an unbalanced fan. Ensure that the correct duct size is being used according to the required specifications in the installation manual. If the duct size has been reduced, it will negatively affect both the suction power and noise level of the range hood. The grease filters may be clogged or dirty. Thoroughly clean or replace the baffle filters. If the issue is not resolved, record a brief video of the sound and send it to support atsupport@thermomate.comThis will allow us to properly diagnose your range hood and suggest any replacement parts if necessary.