Thanks for the tips - I opened the case and now think that the noisy fan is in fact the CASE fan...it is mounted in the back of the case and connects to the motherboard...
I looked in the BIOS setup but there is no setting for the fan...
I see three fans (besides the PSU fan) in the case (Processor, the case fan and another small one on another chip). These are all clean. I wonder is the fan on the case essential?
Oh well, I will just have to wear my noise-canceling headset at the PC!!
Cheers!