To users who don't want to void the warantee:

In effect, there is a way to do this. You need to place something in that will stop the stock fan - an L bracket, or something else like that. Then just affix a fan on top of that / external to the PSU with either longer screws, tape, etc. Granted, the fan below will block some airflow, but it will be quieter, and if you use something like blue painters tape to affix, there's no way that it'd show any voiding