I wouldn't spell a fart sound, haha. I think, if you're going for fart humor, you'd be better off making an analogy to describe the sound. That's a wonderful opportunity to get creative.
This question makes me think of the song, "I Love to Laugh," from Mary Poppins. There may be too many varieties of fart sounds to ever answer your question. Was it an athletic girl's tight-bunned toot, resulting in a flash of embarrassment, or an 80-year-old man's slow, trumpetous raspberry, broadcasting the fact that he is proudly unconcerned about social niceties?
I was in the bath. My butler, Myles Waddle, was on the other side of the door. He's always on hand, you know like one of those Amazon Echos. Anyhow, after a meal quite heavy in vegetable matter and to my embarrassment, I couldn't help but let one out within earshot of the poor fellow. Thus, through the water did the bubbles (before their pingy pop) blurt "wonna-wadder-boddle-woddle". So that's the word I've got for you @jakeybum , granted it's filtered. Oh, and to end this story I have to mention that upon my exit from the bathroom. My trusted Myles was stood ready with a water bottle—convinced I'd requested it.