The sound of an activated but motionless lightsaber in Star Wars movies. Also: nnnnnnnn Reddit
sound of a ruler strummed against a table (ref)
(possibly of imitative origin, but debatable) English language vulgarism most commonly used in reference to flatulence. Onomatopoeia for farts have been suggested in various internet forums in response to questions of people who needed a word to describe the sound: eg. here and here. The suggested words include brrt, braaah, THPPTPHTPHPHHPH (Calvin and Hobbes), phhhhhhrt, PPPPPPPPPPPPPPPPPPPPPPPPPPP, pff, prtrtrtrgurtrufnasutututut, prrrt, PFFT!, PHHhhhh..., SPLPLPLLLP, WHOooooffff, poot, prrrrrrrvt, scraeft, ppppppwwarrrrppppp, pllllllllllllllllllllllllloooooooooooaaa..., RRRRRRRIIIIIIIIIIIIIIIIIIIIIIIIIPPPPP, fuuuuuuuuuuuurrrrrrrt, thhhppbbbb, verrrrrrrrrnnnnnntttttt, hooooooooooooooooooooooooonk, pbpbpbpbp, frr frr frrrrrr rampooooooooo ag, pppppppptttttttttttttttttttttttt, flurpppppppppppppppppppppppppppppppppppp... Also, according to many responders it depends on the fart (farting can make many possible sounds).
sound of the goal light and siren in a hockey game (ref)
sports hockey goal alarm siren tone
sound of a crying dog. from this book: 
Click here for more dog vocalizations
sound of a propeller plane flying overhead (ref)
Sound of a broken power steering pump in a car Cartalk
The sound of an activated but motionless lightsaber in Star Wars movies.
Reddit
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