I just bought a used Novation Nova laptop synth and am having a little problem.

There's a quite but clearly audible crackling/static type sound the can be heard when any of the Nova's outputs are connected. The synth sounds fine, and easily overpowers the noise when played at even low volumes, but the noise remains. The strangest thing is that the noise doesn't increase or decrease in volume with the master level; it always stays at the same level.

I've tried hooking the synth up through different rigs, different amps, made sure wires weren't crossing, etc. etc.

Anyone have any idea what could be causing this noise?