Original sentence: If the motor stays on for long, it makes a piercing hiss.The firing is often very brief, a few thousandths of a second, and usually sounds like a pebble thrown against a metal trash can
Edited sentence: The firing is often very brief, a few thousandths of a second, and  - if operated in air - would sound like a pebble thrown against a metal trash can; if on for long, it would make a piercing hiss