Fan makes loud humming only on first starup of the day

Wintermoons1215

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May 22, 2015
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A few days ago I started up my computer and it made a loud humming or buzzing noise. I opened the case and the noise stopped. The next day the same thing happened--I turned on my computer to this humming noise, only when i opened up the side panel this time the noise didn't stop. So i shut down my computer and turned it back on to no noise. I've figured out it's either my back exhaust fan or my CPU heatsink fan, but that still doesn't explain why the sound stops when I restart the system. It only happens after the deskstop has been off overnight and I'm just starting it up that first time. That in particular is what is what has me the most puzzled and making this post.

Visually nothing about those two fans seems to be off and they seem to be firmly in place, both when they're on and off. And the system starts up perfectly fine even when the noise is happening.
 
Solution
VERY likely this is the first warning that the bearings of your fan are starting to wear out. You should use what you observe to identify which fan it is, and plan its replacement. For example, after it has been shut down overnight you could disconnect the rear fan and then start up. If there is no odd noise, plug that fan back in and see if it makes a noise as it starts.

So, Why? When bearings get worn partly, the clearance between bearing inner diameter and fan shaft outer diameter gets a bit too large. This clearance can change with temperature, and often the clearance is larger when the fan is cool. So when you first start up, the fan shaft rattles around in the bearing for a minute or so and simultaneously gets warmer, reducing...
VERY likely this is the first warning that the bearings of your fan are starting to wear out. You should use what you observe to identify which fan it is, and plan its replacement. For example, after it has been shut down overnight you could disconnect the rear fan and then start up. If there is no odd noise, plug that fan back in and see if it makes a noise as it starts.

So, Why? When bearings get worn partly, the clearance between bearing inner diameter and fan shaft outer diameter gets a bit too large. This clearance can change with temperature, and often the clearance is larger when the fan is cool. So when you first start up, the fan shaft rattles around in the bearing for a minute or so and simultaneously gets warmer, reducing the excess clearance. Eventually the "rattling around" effect disappears until the system is shut down and the fan parts cool off. As the fan wears out more, the effect will last longer on start-up. Eventually the noise won't stop. And after that if you don't replace the fan, it will seize up entirely and stop working. That is bad for case ventilation fans. It can be catastrophic for CPU cooling fans, which is why most mobos have special checks on CPU cooling fans and temperatures.
 
Solution
Thank you for the info! I have identified it is the back exhaust fan--just now I started up my system (the noise happened as usual) opened up the case and lightly touched the center parts of the fans to stop them (probably not the best idea). When I touched the back fan and let it go again the sound didn't happen anymore.

Though I am wondering if it could it still be temperature related since all this happens in a short span of time? The initial startup and me restarting the system (or, just now, touching the fan to pause it) to make the noise stop happens within a minute of two of each other, would the fan parts have enough time to warm up?

Regardless I'll be replacing the fan anyway, thankfully a back fan is a much cheaper fix then a CPU heatsink.
 
Yes, this change to silent can happen in a very few minutes when the problem first appears. The heating effect that causes the clearances to change needs to happen right where the fan shaft runs inside the bearing. And that is exactly where the heat is being generated because of friction at that contact point.