A while ago, I was playing some music from my Dell XP laptop (Latitude), and noticed there was an irregular buzz coming from the speakers. Viewed on an oscilloscope, there is low volume white noise, then about every second, a short burst of around 50 Hz noise, higher volume.
With a bit of investigation, I found that unplugging the laptop from its power pack stopped the noise completely.
I then tried to smooth out the supply voltage of the power pack by placing a few large capacitors on the rails. It seemed the more capacitance I added, the more noise I got.
I don't know what else to do, or even if it is the power pack, and not the laptop making the noises when it is on charge.
Any help is much appreciated.