Strange CPU Fan Behaviour

[chrislev]

My System:

AMD 64 3000+ (standard heatsink/fan)
ASUS K8V-X Motherboard
Standard Mesh Case
80mm Akasa Quiet Exhaust Fan

As part of my upgrade to my system I've been trying to make my PC quieter. I was intrigued by my new AMD64 fan, it seems to vary its speed automatically from 3000rpm to about 4500rpm depending on CPU temp. Is this a motherboard feature (I have Cool n Quiet turn off, this is something different) or is it built into the fan?

My system runs at about 39C CPU and 35C case ~4200rpm CPU fan when idle. The strange behaviour I'm noticing is that as soon as i open my case (even just a little bit), the CPU fan speed plummets to 3200rpm in about 5-10secs. You can actually hear it winding down. This is very strange seen as the CPU temp begins to drop but only after a minute or so as you would expect. What is causing this? Id very much like my fan to run at 3200rpm becuase it really does make a difference. Its totally confused me, I dont know why my fan is reacting like this!

BTW I do NOT have the case open microswitch connected to the motherboard.

Cheers in advance,
Chris.

[fatboyjim]

It will be a BIOS setting, which sets your fan RPM according to CPU Temperature

I have the same setting on my Shuttle BIOS

It is completely different to AMD CoolNQuiet

Have a look round in your BIOS to see what you can find