The first place I would look at is the volume pot. It's not uncommon for the pot to get dirty thereby causing a popping or crackling sound. Move the knob around back and forth a few times and see if that is causing it.

If not, cleaning your contacts certainly couldn't hurt, although I doubt that's the problem unless you've been messing with your wires a lot recently. If it is the wires, it's the connection, not the wire itself.

Are you getting the distortion out of both speakers or only one channel? Figure this out as it will help to determine where the problem lies.

Hope this helps.