However, this is the word for "swan" itself, not the sound it makes. Swans don't have a very distinctive call, and their vocalizations are often described as a "trumpeting" or "whistling" sound.
So, while there is no specific Hindi word for the sound a swan makes, you could use descriptive words like:
* हंसने की आवाज (pronounced "hansne ki aawaz") - The sound of a swan's call
* शंखनाद (pronounced "shanknaad") - Trumpet sound
* सीटी बजाने की आवाज (pronounced "siiti bajane ki aawaz") - Whistling sound