Humorous:
* "The old car coughed and sputtered, sounding like a fart caught in a rusty muffler."
* "After eating the chili, my stomach felt like a drum full of nervous farts."
* "His joke landed with a thud, like a fart in a vacuum."
More descriptive:
* "The wind whistled through the canyon, sounding like a giant, mournful fart."
* "The burst of laughter from the crowd erupted like a sudden, explosive fart."
Remember, similes are meant to create vivid imagery. Choose the one that best suits the tone and context of your writing!