Diagnosis
The LG LMV2031ST is designed for high-CFM extraction, but "crazy loud" noise—specifically a rattling, high-pitched whine, or heavy vibration—indicates an airflow restriction or a mechanical installation error. When the fan is set to High, the motor reaches its maximum RPM. If the air cannot escape the unit efficiently, it creates "static pressure," which causes the air to bounce back toward the motor, resulting in a loud, booming resonance. This is most commonly caused by the installation of a "recirculating" filter while the unit is still configured for "external venting," or vice versa. If you have the microwave set to vent outdoors but the external wall damper is stuck closed, the fan is essentially fighting a wall of air. Additionally, if you used flexible aluminum "dryer-style" ducting instead of rigid galvanized steel, the air creates turbulent eddies that vibrate the thin foil, amplifying the noise across the entire kitchen. You can confirm this by removing the grease filters; if the noise drops significantly, the issue is an external blockage or ducting restriction.
How to Fix It
To resolve the noise issue, you must ensure the airflow path is unobstructed and the mechanical components are seated correctly. Follow these steps:
- Inspect the External Wall Cap: Go outside to the vent termination point. Manually push the plastic or metal damper flaps to ensure they swing freely. If they are stuck due to paint or debris, the fan will roar because the air has nowhere to go.
- Verify Duct Material: Replace any flexible accordion-style ducting with rigid 3 1/4" x 10" rectangular galvanized steel pipe. Flexible ducts create friction and turbulence that increase noise by up to 15 decibels.
- Check the Damper Installation: Remove the microwave from the mounting bracket. Check the rear exhaust port. Ensure the internal damper (the flap inside the microwave) is not bent or obstructed by a screw head or a piece of cardboard packaging left over from shipping.
- Confirm Venting Configuration: If you are venting indoors (recirculating), ensure the charcoal filter is installed correctly and that the diverter is set to "Room" rather than "Outside." If you are venting outside, ensure the charcoal filter has been removed, as it creates unnecessary resistance on the High setting.
- Tighten Mounting Screws: Ensure the mounting bolts securing the unit to the upper cabinet are tight. If the unit is loose, the high-speed vibration of the fan will resonate through the cabinetry, acting like a speaker box.
If That Didn't Work
- Check for Impeller Obstruction: Remove the top grille and inspect the blower wheel (impeller). A small piece of packing foam or a stray wire can get lodged in the fan blades, causing an imbalance that creates a loud humming or rattling sound at high speeds.
- Verify Power Supply Stability: If the noise is a loud electrical hum rather than air turbulence, check the voltage at the outlet. Low voltage can cause the blower motor to struggle and vibrate excessively.
- Inspect the Blower Motor Bearings: If the noise is a high-pitched screech, the motor bearings may have been damaged during shipping. This requires a replacement of the blower motor assembly (Part No. LG-BLW-2031).
- Check for Cabinet Resonance: Temporarily remove the microwave and run the fan on high while the unit is sitting on a counter. If the noise disappears, your upper cabinets are vibrating. Add rubber isolation strips between the microwave and the cabinet.
Heads Up
- Avoid Foil Tape: Never use duct tape on the vents; use high-temperature aluminum foil tape to seal joints to prevent air leaks that cause whistling noises.
- Filter Maintenance: Clean the aluminum grease filters every 30 days. Clogged filters increase static pressure, which makes the fan work harder and sound louder.
- Duct Diameter: Ensure your external ducting is exactly 3 1/4" x 10". Reducing the size of the duct to fit a smaller hole will create a "bottleneck" effect, significantly increasing noise levels on the High setting.