Just for the record…
I was browsing through a load of stuff trying to find details of how to repair a noisy fan on an ACER 4650 series laptop and found this thread.
I bought a new fan through acerspareparts.co.uk
The full service manual for the acer 4561 for free is here
It’s in three parts and part one details how to get the case apart.
You open the case from the top/keyboard side, don’t remove any screws from underneath the laptop to get to the fan…it’s not necessary.
Basically on page 68 of the service manual it says “how to remove the thermal and blue tooth module”. The Fan is underneath this flimsy bit of plastic. Only dismantle whatever is needed to lift this in order to clean or replace the fan. Which means you take the plastic case off the power buttons above the keyboard, unscrew the keyboard and flip it back out of the way, and then remove a flat bit of black plastic that covers the fan. (sections like removing the LCD screen can be bypassed.)
OK when you lift the plastic you’ll see the fan. Encrusted with dirt and fluff probably. remove the two screws holding the fan as it says in the service manual and unplug it. Clean the fan and the little heatsink thing that looks like a tiny car radiator. Use a hoover and brush, but do take some static precautions just in case. The small car radiator style heatsink is very easy to block completely with fluff and that is pobably what your problem is. Reassemble the whole thing as in the service manual, going backwards of course!
You will find that the fan seldom turns on when you are on battery, but when it does it’s real quiet.
If it’s still very noisy like a rattly old train, then buy a new fan.
You can always email me if you get stuck!