Motordyne

If the hiss is not directly caused by flow past partially closed valves, its definitely a form of rasp. Rasp comes in many different characters and volumes.

Some sound like a fart.
Some sound like tinging.
Some sound like a buzzing.
Some sound like a rattle snake.
Some sound like a hiss.
Some can actually sound good where it adds character to the exhaust.

All rasp is caused by standing waves and reflections of shock waves inside the exhaust.

It all depends on the design.