where is ultrasound produced un nature?

Many animals—such as dogs, cats, dolphins,mice—have an upper frequency limit that is higher than that of the human ear and thus can hear ultrasound.

Bats use ultrasounds to navigate in the darkness.

Bats use a variety of ultrasonic ranging techniques to detect their prey. They can detect frequencies beyond 100 kHz, possibly up to 200 kHz. 

Many insects have good ultrasonic hearing and most of these are nocturnal insects listening for echolocating bats. This includes many groups of mths, beetles etc. Upon hearing a bat

Dogs can hear sound at higher frequencies than humans can. A dog whistle xploits this by emitting a high frequency sound to call to a dog. Many dog whistles emit sound in the upper audible range of humans, but some, such as the silent whistle, emit ultrasound at a frequency in the range 18–22 kHz.