I've had the HP Compaq nx6110 for over 4 years now. I absolutely love it, and my only gripe with it was the painfully noisy fan. Searching for a solution on the internet was futile, it seems that this problem was 'By Design' to prevent over-heating. However, it was pretty obvious that the trip points were too conservative which caused the fan to run at full blast most of the times.

I finally managed to fix it today.
Downloaded the service manual from HP and opened up the case. I proceeded to remove the fan and used a brush to remove the dust stuck to the blades. The next thing I noticed was a piece of foam stuck between the grill and the fan (perhaps to prevent dust entry from outside through the grill). Now the foam was really dirty and it didn't look like it would allow airflow through it. For a brief moment, I considered cleaning it or replacing it with something similar... but then I just went ahead and removed it completely.

Put everything back and turned on the machine... and guess what... there's no whirring noise anymore !