Does anyone have any ideas why my father's old Dell Optiplex GX260 machine's CPU fan is loud in blowing (not loud as spinning)? We opened it up and found out it is from the CPU fan (the one with green plastic cover. We noticed the interior and fans don't look that dusty/dirty (amazed at that compared to my PCs after all these years).

I checked the CMOS to see if I could get fan readings or control, but didn't see one.

Any ideas? Do we need to replace it? Both PSU and CPU fans seem to be blowing fine. Just CPU fan is LOUD and blowing hard. And the room temperature isn't even hot which is weird! I wonder if this is related to the low voltage battery error in CMOS a few months ago (haven't seen it since then).

I told him not to take the CPU fan parts apart because he might make it worse. ;)

Thank you in advance. :)