low freq. noise: Notebook->Stereo?

Hi,

I've got a problem with the connection from my notebook to the stereo amplifier. Presently I connect the notebook with a cheap cable. One end has a 3.5mm male connector (to the headphone output of the notebook) and the other end has 2 ordinary chinch connectors to the stereo. The cable is ~5 m long and ordinary thin cupric wire.

My problem: If I connect I hear a permanent low frequency noise (could be 50Hz). The noise stays if I play music. The higher I turn up the volume, the louder the noise.

The noise changes its sound if I run the laptop on batteries. It is still present and sounds similar, but you can hear the change.

My question: Is this a known problem and what can I do to get rid of it?

Thank you.
Cord

P.S.: German and English welcome.