my smooth wave makes a loud humming or grinding noise when it runs, not the normal fan sound.

Diagnosis

The loud humming or grinding noise in the Breville Smooth Wave BMO850BSS is typically caused by a failure in the cooling fan assembly or an obstruction in the magnetron's ventilation path. Unlike the standard operational hum of the transformer, a grinding sound indicates a mechanical struggle. This usually happens when the fan motor bearings wear down, causing the blade to wobble and strike the interior plastic housing, or when a foreign object—such as a piece of plastic wrap or a stray food crumb—has bypassed the filters and lodged itself in the fan blades.

To confirm the source, listen closely to the rear and side vents while the unit is running. If the noise fluctuates in pitch as the microwave heats up, it is likely a thermal expansion issue where the fan blade is rubbing against its mount. If the noise is a constant, rhythmic "clacking," it is almost certainly a physical obstruction. If the sound is a deep, vibrating drone that shakes the chassis, the fan motor mounts have likely perished, allowing the motor to vibrate against the metal frame.

How to Fix It

To resolve this, you must clear the ventilation system and check for mechanical interference. Follow these steps precisely:

1. Complete Power Down: Unplug the BMO850BSS from the wall outlet. Do not simply turn it off at the display; the high-voltage capacitor inside can hold a charge, so ensure the unit has been unplugged for at least 10 minutes before proceeding.
2. Exterior Vent Clearing: Use a high-powered vacuum with a narrow crevice tool. Run the nozzle along the rear ventilation grille and the side slats. Focus specifically on the lower rear corner where dust tends to accumulate and clog the intake.
3. Compressed Air Purge: Use a can of compressed air (duster) to blow short, sharp bursts into the vents. Aim for the fan housing to dislodge any debris that the vacuum couldn't reach.
4. Fan Blade Inspection: Using a flashlight, peer through the rear vents. If you see a visible obstruction (like a piece of packaging), use a pair of long-reach needle-nose pliers to carefully extract it. Do not push the object further into the unit.
5. Leveling Check: Ensure the microwave is on a perfectly flat, hard surface. If the unit is tilted, the fan motor shaft can sit off-center, causing a grinding noise. Use a spirit level and adjust the feet if necessary.

If That Didn't Work

If the cleaning process does not silence the unit, the issue is internal and requires more advanced intervention:

• Bearing Lubrication: If the grinding is caused by dry bearings, the fan motor needs a drop of high-temperature synthetic lubricant. This requires removing the outer chassis to access the fan motor shaft. Apply a single drop of silicone-based lubricant to the motor bearing; avoid using WD-40 as it can attract more dust and degrade over time.
• Fan Blade Replacement: If the plastic blades are warped or chipped, they will create an imbalance that sounds like a grinding noise. You will need to source a replacement fan blade assembly specifically for the BMO850 series and swap it out.
• Magnetron Fan Alignment: The magnetron has its own dedicated cooling fan. If the mounting screws have vibrated loose, the fan will rattle. Tighten the mounting brackets using a Phillips #2 screwdriver to ensure the motor is seated firmly against the chassis.
• Transformer Mount Inspection: If the sound is more of a "hum" than a "grind," check the high-voltage transformer mounts. If the rubber dampeners have hardened or cracked, the transformer's 60Hz vibration will transfer directly to the metal shell, amplifying the noise. Replacing these rubber grommets will restore quiet operation.

Heads Up

Keep these maintenance tips in mind to prevent future noise issues:

• Clearance Requirements: Ensure there is at least 4 inches of open space behind the unit and 2 inches on the sides. Restricting airflow forces the fan to work harder, leading to premature bearing failure.
• Avoid Steam Build-up: When heating very moist foods, ensure the vent is not blocked by towels or other kitchen items, as excessive moisture can lead to corrosion on the fan motor shaft.
• Warning on Capacitors: Never open the main chassis unless you are trained in discharging high-voltage capacitors. The BMO850BSS contains components that can deliver a lethal shock even when unplugged.
• Regular Dusting: Vacuum the rear vents once every three months to prevent "dust bunnies" from migrating into the fan housing.