How do I stop my fridge from making a loud humming noise?

Check if it's level first - a fridge that isn't level vibrates like crazy. Adjust the front feet. Also pull it out and clean the condenser coils, dusty coils make the compressor work harder and louder.

Mine did this and it was the fan hitting ice buildup in the freezer. Defrosted it fully for 24h and the noise was gone.

Both of those are solid suggestions to try first. Beyond the leveling and coil cleaning already mentioned, if the noise persists after defrosting, it's worth listening carefully to what the sound actually is - a grinding or rattling usually points to a fan motor failing, while a continuous hum might be the compressor working overtime due to a refrigerant leak or thermostat issue. At six years old your fridge is definitely still in the range where a repair makes sense, so if none of the DIY fixes work, a technician visit would tell you pretty quickly whether it's worth fixing or if you're looking at replacement costs anyway.

Those two answers pretty much cover the common stuff. I'd add that sometimes it's the evaporator fan motor itself starting to go out - if you hear a grinding or rattling sound specifically, that's usually it. Before you drop money on a tech visit, definitely try the level check and coil cleaning first since both are free and fix most cases. If it's still loud after that, yeah you're probably looking at a service call.

I had a similar issue with my fridge a couple years back and it turned out to be a combination of things, which is probably why you're getting that persistent hum. The suggestions about leveling and cleaning the condenser coils are spot on - I'd definitely start there since those are the easiest fixes. But what nobody's mentioned yet is the compressor itself might just be getting tired. At 6 years old your fridge is still fairly young, but compressors can start getting noisier as they age, especially if they've been working harder than usual (which happens when coils are dirty or the fridge isn't level, so fix those first).

Here's what I'd do: first, get a level and check if the front is tilted up just slightly - most fridges need that tilt to drain properly, but too much angle causes vibration. Then pull it out from the wall and look at those coils in the back. They're probably pretty dusty by now. I use a vacuum with a brush attachment since my beekeeping setup gets everything covered in pollen anyway, so I'm used to dusty equipment. While you're back there, also check if there's any ice buildup visible anywhere, since that fan blade hitting ice will make a grinding noise that's pretty unmistakable. If after doing all that the humming hasn't changed, and especially if you're hearing grinding or rattling rather than just a steady hum, then yeah, the fan motor or compressor is likely starting to fail and you'd want to call someone out. But give the basic maintenance a shot first - surprised how often that solves it.

The compressor itself might be failing, which is the thing nobody's mentioning yet. If you've ruled out the level, cleaned the coils, and the freezer isn't iced up, listen to whether the hum changes pitch or if there's a clicking sound when the compressor cycles on and off. That's usually a sign the compressor is reaching end of life, and honestly at 6 years old it's getting up there depending on the brand.

One pitfall though - don't go poking around inside the fridge or freezer compartment without knowing what you're doing. The refrigerant lines are under pressure and if you puncture one trying to access the evaporator fan or compressor, you're looking at a way bigger bill than a service call. I've seen people try to DIY this stuff and make it worse. Coil cleaning from the outside and defrosting are safe, but anything involving internal components should probably go to a tech.

If it's still humming loud after trying the level and defrost thing, just bite the bullet and call someone. A compressor replacement on a 6-year-old fridge usually isn't worth it economically, so that's something a technician will tell you anyway. Better to know now than keep running a failing unit and have it die completely.

The compressor itself can wear out after 6 years and start running constantly, which is way different from a one-time fix like leveling or defrosting. Before you assume it's something simple, listen closely to whether the humming changes pitch or stops during the day - if it's basically non-stop, that compressor might just be getting old and you're probably looking at a tech visit or replacement.