Audio distotion from laptops

I’m not quite sure if anyone can help on this one as it’s a bit strange.

I have been installing AV boardrooms for over 10 years now. They consist of pretty much the same kit.

Data Projector
Amplifier & Speakers
VGA & Audio switcher to switch between a fixed PC and a couple of laptop plug in points.

When the PC, running XP and using Windows Media Player (WMP) to play audio, is routed through the switcher to the projector and amplifier the audio is fine. However, when a laptop is connected and routed the same way there is a clock pulse distortion on the audio.

Things to note,
1. It is always present when the audio is connected.
2. It is present when the VGA only is connected and WMP is open. If I close WMP is goes away.
3. It is an unbalanced signal into an unbalanced switcher. I have put in an unbalanced to balanced transformer in the signal path but it makes little difference.
4. This problem only happens when the laptop is connected to the mains. I suspect that I never had it before because all my previous laptops didn’t have an earth pin. My current one does.
5. The PC is a line out socket. The laptop is a headphone socket.

Any wise words out there?

Ta,