How to tell if a fan is working properly?

Hi,

I have my casing open for some time, and I used to wonder if the noise the CPU fan was making is really louder than it is supposed to be. It is a loud even hum, which doesn't cause the case to vibrate but you could easily tell that the PC's on from several meters away.

I have a P4 3.0 CPU and a ASUS P4P800 motherboard. I do recall once having to remove an extra optical drive because the CPU fan refused to work with two HDDs and two DVD-ROM drives.

Also, I have noticed at times when I run ASUS Probe that it occasionally warns me that a fan (either the CPU or chassis) has negative RPM ("CPU fan below threshold!" ), which is strange because I can clearly see both sets of fans spinning (in the same manner as described above) from where I sit.

The same ASUS Probe software currently reports my CPU and Motherboard temps at 31 and 46 degrees celsius, respectively.

Does it look like I need to do something with the fan :?:

should I be worried that Probe from time to time reports negative RPMs for the fan, then?

just what i thought. you're right, the monitor does pick up the correct rate after a few seconds; but admittedly the no cpu alarm is quite distracting.