The Basics
Refrigerators are kind of like a library; they're usually pretty quiet these days. An occasional whirr, thump, or hum is about all. So, noises can be alarming.
Normal Knocking
Let's start with some normal knocking-type sounds. Knocking noises are a lot less common for a refrigerator than humming, but there are normal processes that cause them.
Ice cubes falling into the ice bucket will sound like a knocking noise.
The compressor starts and stops with perhaps a single knock.
If you have a dual evaporator system, you might hear the three-way valve cycling as it diverts refrigerant from the freezer evaporator (the cold thing in the freezer) to the fresh food compartment evaporator (which keeps the fresh food compartment cold).
Sometimes, the water dispenser can make a single knock when it starts or stops.
Some device or accessory may have been inadvertently turned on, like an ice maker that you don't use and may not even have a water supply. It's making a sound that is normal but abnormal to you.
The Additional Information section (link will open in a new tab) below contains a long video from LG that goes into their refrigerator sound library. You might want to use it for confirmation. Samsung has a video below as well.
When does it Knock?
When you hear the knocking sound, try to notice anything else that may be going on at the same time.
- Is there new ice in the ice bucket? The Ice maker just produced ice. (Normal)
- Can you hear water trickling from the defrost drain? The defrost cycle might have just ended, and the compressor is working harder. (normal)
- Did you just dispense water? The inlet valve may be banging, or there may be a water hammer.
- Did you just shake the fridge? Sometimes, the compressor will knock a bit when that happens if it is running. (normal)
These are just some examples of operations that might help you figure out what part has a problem.
Where Is the Knock Coming From?
The location of the knock can give you important clues.
Sounds from down low in the rear of the machine are likely condenser fan or compressor-related.
Sounds from the freezer area may be related to the evaporator fan or ice maker.
Knocking from inside the door where the ice and water dispenser is located can be from the dispenser or related to the ice maker. The in-door ice maker uses a tilt and twist tray, and the drive mechanism for this can make knocking noises, especially when going bad.
Some freezer ice makers will make considerable noise when jammed with ice as the drive is designed to slip and may make loud thumps when it does. Here's a video of a craft-type ice maker knocking.
Sounds from the rear of the unit near the center could be damper-related or from a bottom freezer ice maker.
You may want to start with some of the suggested items above in the list below.
Causes
Ice makers are normally kind of noisy, but knocking or thumping is usually a sign of a problem.
- Turn your ice maker off, and then see if the noise disappears.
- The same applies even if you don't ever use your ice maker. Check if it was accidentally turned on. It will make a knocking sound as it tries to cycle.
- Run a test cycle on your ice maker to see if you hear the sounds.
- A jammed or iced-up Ice maker will be noisy, so check it thoroughly for stuck ice.
If it checks ok, go to the next item.
While not as prone to making knocking sounds, the evaporator fan blades may be hitting ice buildup. Your defrost system may be failing or out of adjustment. Open the freezer (usually) and see if you hear knocking from the back area of the freezer.
Clear away the ice if present and check for any other obstructions. Ensure the freezer's air vents are clear of food and stored items for good air circulation.
If the evaporator fan is running well, go on.
Check this, especially if you hear noise from the rear of the fridge down low.
The condenser fan might make a knocking if a part has come loose and is interfering with the blades (like an evaporator/condensate drain tube).
You might also have something that worked its way under the fridge and is now in the blades. Think of small cat toys as an example.
Or items that an industrious toddler has pushed under the fridge.
If ok, go on.
This isn't necessarily from the fridge proper, but it can be noisy, and it may be from piping in the wall immediately behind the fridge.
It could be related to the cycling of the ice makers (usually, the flow is too slow, but it could happen) or water dispensing. A quick shutting solenoid valve controlling these things can make it happen.
You might want to see if there are other places in the home that shutting a valve quickly causes this. (personal experience of a knock sounding through the whole home when a washing machine would finish filling or was periodically spraying water in a rinse cycle). The pipes behind the fridge could be making a sound in response.
No water hammer signs. Go to the next item.
Kind of unlikely because the damper motor is so small and low-powered; a stuck or jammed cold air damper between the freezer and the fresh food (refrigerator) compartment could make a knocking sound. Probably not very loud, maybe a few knocks or thumps in a row. It can be normal too if it isn't very loud.
No damper sounds, go to the next item.
The potentially bad one. Sometimes, a compressor will make a knocking sound trying to start if it has been suddenly stopped mid-cycle, like by a power failure or a circuit breaker tripping. Often, this will go away, and the unit will start, given time to try again.
Keep in mind that if the fridge is moved while running, the compressor may bang against its housing. This should go away within no more than 60-90 seconds.
If the knocking is frequent and lasts for more than an hour or so at the beginning of a cooling cycle, you should have your unit evaluated by a trusted professional. They can better determine if it is a sign of failure or not.
Additional Information
Very thorough video by LG dealing with refrigerator noises. Long but detailed with good recordings. 13 minutes in or so is where the abnormal noises start.
Shorter, not as thorough video from LG. Probably a newer version. Doesn't let you repeat sound samples easily.
Here is a Samsung Refrigerator Sound Video
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