Noise in audio out when charging laptop

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http://askubuntu.com – I normally use a mini jack cable to connect the audio out of my laptop to my guitar amp to listen to music. When the computer is on battery I have no problem whatsover with the sound quality. However, the second I plug in the charger to the laptop I get a lot of noise in the guitar amp. I have Windows 7 on dual boot and this problem doesn't happens there, which allows me to conclude that this is software problem of some sort. Any idea how to solve it? Thanks. (HowTos)