Adveser Messages: 434 Registered: July 2009 Location: USA
Illuminati (1st Degree)
Gently push on the speaker's cone and make sure they don't sound like they are full of broken glass.
If that isn't it, I would look at the dials (potentiometers) and move them around a little to see if you can reproduce the noise. The noise you describe is the classic balance slider's metal leads wearing thin.