Sounds like something is arcing. Check you plug wires first to make sure they're secure and haven't rubbed or burnt trough on the sides of the wire anywhere along it's path. Make sure the coil module is secure in it's "nest". If the external stuff checks, then go to the rotor and cap and see if there are signs of arcing there, although it should be more difficult to hear that than something external. Check the cap for cracks and carbon arcing as well.

Think about recent changes, or other actions under the hood. Any changes made with anything that could be around the ignition wiring. Back track from there.