We had a similar problem, but it was obvious what was wrong because the fan started screeching in the middle of a hurricane, so you can imagine how loud it must have been.
We had to dismantle the parts inside the frig that hid the fan (not a big job really) and find and remove the fan. By getting the designation number off the fan,we were able to order a new one from Ebay for about $60, and install it. It's lasted over a year (through a 2nd hurricane) without a peep.