Ignoramus5827 wrote:
I have a couple of devices with very noisy and annoying fans. One is
an old laptop and another is a car inverter. In both, fans make a very
annoying, inconsistent, and rattling sound. Before taking those things
apart etc, I wanted to double check what is the typical cause of this
and how to deal with them.

i

Sounds like a sleeve bearing fan and the bearings have gone dry. Quite
often they can be rejuvenated by re-lubricating the bearing. I have a
couple in my PCs and they require treatment every couple of years.
Normally you can left the label off the centre and you'll find the end
of the shaft with a small circlip, so far all the circlips have been
plastic, so remove the circlip and the fan and shaft can be removed.
Clean the shaft and bearing with some tissue , add a drop of oil, I use
airline oil, and re-assemble and all should be well again. BTW one fan
has a rubber bung sealing the end of the shaft which needs to be removed.