I have had my Vostro 400 for several years now and was never really bothered by the fan noise until recently. I mostly used it for homework (when still in college), web browsing, etc, but recently I upgraded the graphics card and have started working from home using CAD software (SolidWorks 2012). This puts it under considerable load the entire time I'm working.
It didn't take long for the constant fan noise from the CPU cooler fan to get very annoying! I used Open Hardware Monitor (OHM) to see that when under full load the CPU cooler fan was spinning nearly 4200 rpm. After some looking around I found a suggestion to change out the stock CPU cooler fan which uses pulse width modulation (PWM) to controll the speed with this fan:
All I had to do for the install was unclip the stock CPU cooler fan and snap the Coolink fan in place. I did take the opportunity to vacuum the accumulated dust out of the CPU cooler while the fan was off. After putting everything back together I restarted the computer and was amazed by how quiet it was! Even under 100% load on the CPU and GPU the fan is barely audible. OHM says the new fan is only spinning about 2200 rpm at full load.
For anyone who has a noisy desktop, I highly recommend changing out the offending fan! I am amazed by how much easier it is to concentrate while working. I know it sounds funny, but I feel better at the end of the day now where before after listening to the fan noise all day I would have a bit of a headache.