I've got a Macbook Pro which I use with some external monitor speakers, but I have a problem. Whenever the sound is quiet (such as a quiet section in a song, dvd, anything) I can hear a hissing sound, and when the sound goes almost silent, it stops, and then comes back the moment anything makes a noise. I could deal with the hissing if it didn't make itself obvious by being intermittent. I have tried different speakers, too, and I had the same problem. I can't seem to work out why this is happening. Any ideas?