Quiet sinus noise when playing a sound on my laptop

I am recording my tracks from Renoise over an external mixer on minidisc (I am using some external synths). My soundcard is a Focusrite Saffire 6 (Asio).

The last recording I recognized a quiet mid frequency sinus noise at the beginning and the end of the track (where the track is quiet) and started to investigate where it comes from. When I am opening Renoise with a clean new song (no plugins, nothing loaded, no audio inputs, just plain clean Renoise start), then I can hear this sound. I also can hear the sound in other software, but only if I am playing a sound actively; e.g. in Winamp (after stopping, the sound disappears after about 5 seconds) or when I am playing an empty sound in Audacity.

Does anybody has an idea why this sinus sound appears, when I play any sound?

As an example please refer to the very end of the track, after the final crash, here:


Well after investigation it turned out, that the tone (exactly at 1000Hz and 2000Hz) comes directly from the Line-Outs of the Soundcard. When I am plugging a headphone at the soundcard, there is not such a noise.
I am quite disappointed of the soundcard hardware.