The sound in question is made by the male, in courtship display – by rubbing its penis against its abdomen.
Science Daily explains this, but shys away from naming the actual body parts. The Telegraph has no such qualms.
This is an example of a process called stridulation, which is basically the act of producing sound by rubbing together any two body parts. It's also how grasshoppers and crickets make their well–known sound – but they do it (according to the Canal & River Trust) by rubbing their back legs against their forewings.
In the words of Wikipedia: "Male grasshoppers spend much of the day stridulating."
Enough said, I think.
© Haydn Thompson 2024