Sparks and hissing means high voltage has found a path for arcing.
Possibilities:
* Wire insulation is perforated.
* Painted metal has lost insulated quality
* Metallic objects contaminating surfaces
* Dust and dirt has built up, especially carbonized dust
* High humidity
* A fault in circuitry has created high voltage of opposite polarity nearby
The quickest way to solve it is to spot exactly where the arcing is occuring, and move the arcing wire away from nearby objects.
It will help if you clean away dust and dirt.
Observe in a dark room to make it easier to spot.
You should wear thick rubber gloves if you touch high voltage. Or else remove power.
Dry air will reduce the opportunity for arcing.
If the arcing is inside a tube or crt then it means a bad (gassy) tube and you'll have to replace it.