Fläktar dras igång när jag inte gör något krävande på Lenovo Yoga Slim

There are multiple things that can run in the background and cause this - Windows indexing service, your browser, antivirus, etc. So as mrqaffe suggested you should check Task Manager or even better Resource Monitor which should give you idea what is happening.

There is a chance that once you open Task Manager or Resource Monitor you don't find anything. This can happen if what ever worked in the background detected your activity and stopped the processing (regular applications working in "nice" mode). Also, there are reports recently of malware that installs crypto miners and these miners are known to turn off when you run Task Manager or some other tool in order to avoid detection. So running a good antivirus and MalwareBytes is recommended to check this.