Some Supermicro systems I work on recently starting having very noisy fans. The systems are Supermicro 1U 6013P-8 servers. They each have two squirrel cage fans. The fan has Supermicro labels that read "fan-0066" and "672042610488". The OEM label on the fan reads "Nidec GAMMA30 Model A35317-58 ABE 12 V DC 1.05 A 3Y26M2 NIDEC CORPORATION MADE IN CHINA". Here's a picture
The two halves of the fan housing are held together by two screws and three plastic clips on the edges of the housing. Taking this apart is pretty easy. Once you get it apart it's clear that the noise is being caused by the cage rubbing against the housing.
The cage is held to the motor by a retaining ring that sits in a groove on the end cage shaft and prevents the shaft from pulling out of the motor/bearing. This end of the shaft and the retaining ring are behind the yellow sticker pictured above.
Here's what it looks like when you peel back the sticker on a failing fan
All that rust dust was ground off the retaining ring and shaft over years of use. If you look closely in the picture you can see the retaining ring has popped completely off. Here is what the retaining ring looks like
With the retaining ring off, you can pull the cage out of the motor. Here is what it looks like from a couple angles, a view of the groove in the shaft the retaining ring sits in, and what the bearing looks like when it's not covered in dust.
In the above pictures you can see on the cage side of the shaft there is a small spring to keep the cage off of the motor. With a retaining ring in place, that spring would keep the cage/shaft/ring pressed up against the bearing, and centered in the housing. So when the ring wears out and pops off, the cage moves slightly away from the intake side of the housing and starts rubbing on the other side of the housing. Now fortunately, due to the magnet in the cage being attracted to the motor, it wants to stay pretty close to where it should be, but it's still enough to make a huge amount of noice, slow the fan down, and probably eventually cause the fan to lock up.
You can buy replacement fans online for ~$20USD, but they will eventually have the same failure mode. Instead you can replace the retaining ring. I wasn't able to find identical wire rings, but found something even better, an "e-clip" (picture). The diameter of the shaft is 7/64th of an inch. You can buy e-clips in 1/64th increments, but you need one that fits the groove diameter which is less than 7/64ths. How much less? I wasn't able to measure, but using a clip that is 3/32nds would allow for a 1/128th groove depth which is probably about right. I went to Hardwick's and they happened to have a couple e-clips that fit (although they weren't labelled and are hard to measure). They were also able to order other sizes (I'll have to get the name of their supplier to post here), in bags of 20 e-clips for $3. I ordered a bunch as I have ~20 fans to repair.
Here is a picture of the fan with new e-clip
It's working great and isn't making horrible noise. I will probably take one apart after a month or two of use to make sure it's still doing ok. Hopefully this page saves people some money and some fans from the landfill.