SONY VAIO VGN-CS21S/P - the fan swi...

The problem:
the fan on the left side makes a lot of noise (switches on and off every minute 2-3 times for 10-15 seconds), but it starts to do that only when I go online (I tried with WI-FI and mobil internet).
This is why I could not reproduce the phenomenon when I took it back to the shop where I bought.
The noise is just annoying, especially when switching on - I can hear a loud blowing which is very disturbing when concentrating while working.
The shop said that I should set the BIOS. It is not really something I would know (how could they expect that from a woman?) I will call the service tomorrow, but I would like to know more as soon as I can. We have a 3 days changing back guarantee here in Hungary, and I would like to hear ASAP if this is a type-error of this machine.

Any comment about it? Thank you in advance! :-)

Orsi from Budapest (Hungary)