The fan is always on, even when the desktop is rather cool

[Wei Hu]

I have 3 systems in my desktop:
1) When FreeBSD runs, my desktop fans are always running, and this
make annoy noisy.
2) However when Debian runs, the fan eventually stops unless I am
performing a load intensive task.
3) In Windows, the fan is almost always off.
I tried to use acpi and apm, but they are for laptop.(?)
In Freebsd, how can I control the cooling fans or how can the system
turns the fans off when the load is not heavy.
Any help would be greatly appreciated.