Is it normal to hear the MacBook fans all the time even if they run at 2000rpm? I have an early 2011 15” MBP (2.3GHz i7 8GB) and I’m annoyed about the sound of the fans. I know how a defective fan sounds, but these do no sound abnormal, but what what is abnormal is that I hear them even when computer is almost not used.


The fans are running at ~2000rpm and ~64 C.
I do have a SeeThrou Satin Case which I bought from Apple, so I suppose it should have a major effect
No signifiant HDD or CPU activity observed with Activity Monitor
Yes, usually I do keep it on my lap, eventual on bed and have to use a pillow or something else to isolate me from its heat. 


This annoys me because I would like to have a quiet work environment and not having to to start music just to hide the FAN noise.

What can I do to solve the noise problem?
What you can do is use the laptop on a solid underground. Especially if you use a pillow (which where you laptop completely sink in), the pillow is covering the laptop's ventilation holes. And it's even stated in the user manual: 


  Do not push objects into the ventilation openings.


The exhaust is the screen hinge, but the intake is most likely all the other cracks and holes in the case. And if the laptop can't take enough fresh air (since you're covering most of the cracks and holes with your pillow/blanket/lap), the fans have to work very hard to get all the hot air out.
Try to lift up your Mac as you're sitting on your bed, does that improve the activity of the fans?

But no worries, you're laptop won't suffer from the heating. And if it get's too hot, it will shut itself down (to prevent damage), so then you'll know you've gone too far :)

Another thing what makes your Mac blow like a turbine is playing flash-files or converting stuff, but I guess you're not doing that since you haven't got any high CPU or HDD activity.