I tried rebooting (and unchecking “reopen apps at startup”), which helped for about 5 minutes.

I tried closing down apps (which helped; especially keeping Mail closed helped a lot). But even watching a YouTube or playing Music on Google Music caused the CPU fan to start spinning way louder than before.

It quickly came to a point it started driving my crazy and I had a really loud machine which was also sluggish and laggy at some random points (=> not sure if it’s related, but I haven’t had it since).

The thing I tried was a SMC reset, and that did the trick. Since then I haven’t heard the fan on normal load anymore.

Do this (on an Air):

1. Turn off the Mac
2. Be sure it's connected to the Magsafe
3. Hit left shift + ctrl + alt (option) + power (like just one second or something apparently)
4. release the keys
5. and boot normally (hit power button)
6. that should be it