The Fan of the HP Microserver N54L is quite loud, so a replacement would be nice. But there are some pitfalls:

• the pinout of the four pin PWM header is different. Read This Page which pins need to be swapped.
• the new fan needs to run at about 700rpm miniumum! If it runs slower (like only 500) the BIOS gives you a "Fan error" and the system shuts down.

I tried with a Enermax UCTB12P which runs at 500rpm min and the server shuts down... Now i use a Bionic loop fan Bl2-P PWM which works great (the only disadvantage is that you can not use the vibration decoupler...)