Computer fan rattling

Solution
Three possibilities come to mind

1. Some fans have a rubber grommet that silences and dampens vibration ... some do not. You can buy these:

http://www.frozencpu.com/cat/l3/g47/c111/s814/list/p1/Fan_Accessories-Computer_Fan_Silencers-140mm_Fan_Silencers-Page1.html

2. Some fans have a bearing design which allows the blade assembly to move a bit along the shaft. When installed horizontally, and blowing down the blades blowing down push the blade assembly up, and if rpm is just right it can to the up / down rattle thing.

3. Bearing shot
Three possibilities come to mind

1. Some fans have a rubber grommet that silences and dampens vibration ... some do not. You can buy these:

http://www.frozencpu.com/cat/l3/g47/c111/s814/list/p1/Fan_Accessories-Computer_Fan_Silencers-140mm_Fan_Silencers-Page1.html

2. Some fans have a bearing design which allows the blade assembly to move a bit along the shaft. When installed horizontally, and blowing down the blades blowing down push the blade assembly up, and if rpm is just right it can to the up / down rattle thing.

3. Bearing shot
 
Solution
Sounds like you're getting vibration from either the fan not being securely mounted, or perhaps the case has a loose spot. Get more specific with your touches and pin it down. At that point, secure down a screw if it's there, or wedge a foam shim, or whatever it takes.
 
Any decent fan manufacturer will provide them in the box with the fan

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It sounds like the bearing is drying up... Pull the fan and gently lift the sticker on the back side.. then add a few drops of sewing machine oil.. spin it manually until it spins freely again, then place the sticker back over and you should be ok for 6 to 12 months.