That grille is quite restrictive. As Tephras pointed out, removing it will increase the airflow. But he forgot the last step. This increased airflow will allow you to run the fan even slower. With such a low power system, running the fan at 5V will most probably be OK, when that grille is removed.
If you are not afraid of just cutting the grille from the PSU, but removing some small pieces of your case too, then you can mount your PSU fan something like this:
You don't need to do it as drastic as I did.
Just cut some small pieces from the case
so the fan will fit and you can still use the
screws to attach the PSU to the case.
In most PSU's the fan and the internal components are quite close to each other. As impedance on the intake side of a fan generates more noise than impedance at the exhaust side, the effect of it should be greater than just grille removal.
One warning though. Use a sturdy way to attach a fan when you put it on the outside of the case. Screws or rubber/silicone fan vibration isolators are OK. But somebody once reported using double sided tape. When the tape dried out, the fan fell off and the PSU fried.