Noise, Hum and Vibration From a Roof AC Unit

Ball Bearings

• Under-lubricated fan ball bearings can cause a serious ruckus and problems for proper air conditioning function. They could also make the whole unit vibrate. If you hear a shrill grinding noise coming from the unit, power flow to the unit should be turned off. You can remove the fan-housing panel on the rooftop air conditioner and examine the state of the fan component. Grab the blades of the fan and spin it around. The fan should turn freely. If it doesn’t, then the ball bearings might need lubrication. Electrical motor oil can be inserted into the ball bearing ports along the axle. After the oil is worked into the bearings, the fan should spin freely.

Debris

• If foreign objects, such as sticks or trash bags, have made their way into the fan housing, they could bind up in the gears of the fan and cause it to stop. This causes the unit to vibrate from being stuck, and to hum, because the fan motor continues to run. Power should be shut off prior to removing the rooftop air conditioner panel. All debris must be removed from around the fan area. If anything is wrapped around the fan axle, it should be cleared. Spin the fan around to confirm that it turns freely.

Ducts

• While it’s true that foreign objects inside the duct system of the roof air conditioner could cause vibration and noise, it could be the ducts themselves causing the racket. Ducts expand and contract with use, which can cause noise. Also, if the ductwork isn’t secured to a wall or the framework of the building, it can rattle and move around as air flows through it. To stop ductwork noise, you can access the lines through the attic or crawl space and fasten them to the framework with special straps or duct tape.

Vents

• Improperly secured vent covers can cause a clanging, vibrating or rattling noise as the roof unit runs. You can inspect the vent locations and test them for looseness. If the vent is loose, you can tighten the screws with a screwdriver. If the vent continues to loosen, you can remove it and insert lock washers onto the screws to prevent the screws from loosening.