It's actually a breathing technique that some competitors have taken to a higher level. Think of it as a trampoline version of a weightlifter's exhale or a tennis player's groan after hitting a ball.
Canadian coach Dave Ross said he first noticed it in the sport some 15 years ago. Since then, many competitors can be heard whistling while they jump on the trampoline.
"Instead of (exhaling) quietly, they kind of (whistle) as they do it and some are a little louder than others," Ross said.
Bronze medallist Lu Chunlong of China was one of the competitors who could be heard whistling in Friday's men's competition, usually while launching himself in the air.
"It just seems a bit weird that people are making that whistling sound," Ross said. "But I think it's almost like a fad at the moment."
Ross added he finds the practice a "bit annoying."
"I don't train people to do it and I don't have anybody doing it," he said. "But a lot of the international performers are doing it. To me it's not an important part of the performance."