You have added some info that directs us elsewhere. See if this describes your situation.
Symptoms: when first started up after being down for a long time (and hence cooled off), there is a loud fan noise. This continues for a few minutes, maybe even 10 to 15. Then it stops. As long as you keep running it is quiet. If you reboot it stays quiet. If you shut down and re-start very soon, it is still quiet. But if you leave it off for a long time, it is noisy again when you start up.
If THAT is your pattern, you have one fan (usually not more than one, but could be) with bearings so worn they are making this noise when cold. You can identify the failing fan this way. Shut down, make sure the system is cool, and open your case so you can reach the fans. Get a pencil or similar. Start your system. One by one, reach in with the eraser end of the pencil and temporarily stop one fan, then let it go. That will identify the noisy one.
As fans run their bearings wear slowly. After a few years the clearance between fan shaft and bearing sleeve gets large enough that it can "rattle" when your system is cold. But as the fan runs and the bearings heat up, the gap narrows and the rattling stops. Until the next time it's cold, that is. When you hear that patterm it is the first stage of fan bearing failure. It may keep runing that way, with the noise period getting longer, for months or more. But it WILL fail. So plan to change that fan.
If the noise pattern is NOT as I describe above, then it may be something else.