My recently installed bathroom exhaust fan operates louder than expected?

Here are some checklist items to review when experiencing this situation:

1. Remove the fan grill from the ceiling and turn the fan on, make sure it is operating properly; blade is rotating freely, no obvious issues (slow rotation, wobble, rubbing). Make sure the fan motor and or blower are installed securely.
2. Most exhaust fans have a back draft damper installed in the outlet, ensure this damper is opening and closing freely with no obstructions. You will need to remove the fan’s motor/blower to inspect the damper, make sure you turn off the power to fan first.  If the damper is operating correctly reinstall the fan’s motor/blower and turn the power back on.
3. Exhaust fans must be vented through a roof, wall or soffit exhaust vent which has a damper. With the exhaust fan running check the exhaust vent and make sure the damper is opening and closing, check for airflow out of the exhaust vent.
4. Ensure the size of the exhaust duct to the outside is at least the size of the collar diameter that is attached to the fan’s outlet. If the installed fan’s collar is 6” using a reducer and attaching 4” ducting will greatly reduce airflow and cause restrictions.  Same circumstance for 4” collar to 3” duct.
5. Length and type of exhaust ducting installed. Keep any exhaust duct run and elbows or other fittings to a minimum (refer to article: https://laskoproducts.zendesk.com/hc/en-us/articles/4414361059085-How-long-of-a-duct-run-can-I-install-on-my-exhaust-fan-).  Flexible ducting while convenient is highly restrictive, when used ensure it is pulled as tight and smooth as possible and is not compressed or pinched in any way.  Any length of flexible ducting is considered twice as restrictive as smooth rigid ducting.  If flexible ducting is the only option, or if you have an extremely long duct run then consider increasing the diameter used.