laptop to pa connection problem

hey guys, i am new to the forum and was after some advise. i have just bought a PA system and a laptop and want to get into some dj'ing (i was thinking of using virtual dj unless there is a better program out there). however when i connect my laptop to the amp there is a ringing noise in the background when i play mp3's do i need a good sound card to fix this problem? when i hook up other devices to the system there is no ringing (ie a dvd player) so it's something to do with my laptop/soundcard. my laptop is a Benq R55V40 Notebook. nothing is modified, but i bought an internal hdd with a case to make it external. any advise would be appreciated.

Can you be more specific as to this ringing sound? Also what bitrate are these MP3s?

Have you tried using the laptop without the mains adapter plugged in? Does the buzzing sound still exist?

actually i think dave87 may have helped. when i unplug the power supply the noise isn't there. i haven't had a chance to fully test it because it's 1am in australia and i don't wana anoy the neigbors but i'll test it later. if that is the problem is there a something i can do to keep the power supply connected but not have hte noise? thanks

the noise is interference due to problems in the power filtering. the onboard sound on my board would make a clicking noise (like an iPod) only audible when i got my decent headphones.

An external soundcard of decent quality would likely solve your problem, as running on battery power isnt really a long term option i wouldnt have thought.

thanks for your help guys