Hi ch_123,
This could mean that one of the fans is starting to seize. When the PC is idle, the
CPU for example starts to cool down, and therefore your fans also start to run slower.
What you need to do is open the case of your PC, and find out exactly where the noise is coming from, most likely your
CPU. Then use a brush, or preferably an air duster to get rid of the dust that is blocking your fans. The fans are located at the back of your PC, in the centre of your PC (
CPU), and new graphics cards also have them as well.