Old fan was making all sorts of noise. I didn't even realize my Mac had a fan for years until this fan started to go, and then it was obvious as a blue sky.
Overall, it was a pretty easy repair, but a tad trickier than a standard PC laptop due to the tight tolerances and incredibly small parts. If you can put the Mac in a shallow box so no screws get lost in the process, it'll probably help considerably.
The only super tricky part was sliding the new fan's ribbon cable into the slot. I used the spudger and pushed on the only small crease I could find to slide it into the slot. I suggest powering it on while still apart to verify functionality, then power it off and finish stitching everything back together.