I have just bought some Harman Kardon SoundSticks II. When I plug them into my iPhone, they are perfect, however, I want to use them with an airport express and as soon as I plug them in I get a loud buzz/feedback noise. The same happens when I plug in my computer. Can anyone point me in the right direction to get rid of this?

This is from Harman support site First you need to determine where the problem is. Try connecting your speakers to another audio source such as a portable CD player. If you receive the proper sonic results, this will tell you that the problem is in the computer and/or soundcard setup. In this case there are many things to try. 1. Interference may be causing the distortion. Please check all surrounding electrical objects and place the speakers away from the problem object. 2. Ensure that the speaker input jack is pushed all the way into the appropriate jack. Check all the connections and secure them. 3. The volume on your soundcard may be set too high. Decrease the volume in your volume control panel. Also check the volume and tone controls on the speaker. 4. Sound card or audio source material may cause a problem. *note* the quality of the audio source is also a factor (i.e. bad recording). The compatibility of your CD/DVD rom drive, soundcard and software will contribute to the quality of your sound.
Good luck