The sound made by giant tortoises during sexual intercourse. During said intercourse, the male tortoise balances precariously atop the female tortoises shell, and proceeds to penetrate her whilst rocking back and forth on her shell whilst emanating a sound that sounds very much like an elderly couple making love right before they turn to dust.
Tom: Are those tortoises having sex?
Kat: Why yes, it appears as though they are.
Kat: Kind of reminds me of my grandparents.