What i'm basically trying to do is picking a string once and then just slide around and have the amp feeding back.

Kinda like whats going on with this song, http://www.youtube…"]YouTube - Dream Theater - The Ministry of Lost Souls 1/2[/]="http://www.youtube…"]YouTube - Dream Theater - The Ministry of Lost Souls 1/2[/]
JP does a fill 4:10 then comes the feedback thing under the singing.
If anyone has "Chaos in Progress" you can see him do it without the vocals.

I'm just wondering what amp settings, guitar technique or plugins I should use do get this effect.

Regards.