Here are some common options, depending on the intended effect:
* Simple: "burp", "brrp", "brp", "urp"
* More forceful: "blorp", "bloop", "bloosh", "blarg"
* Humorous: "blub", "blat", "bleh"
* Onomatopoeic: "pfffffft", "phhht", "whoosh"
Ultimately, it's up to you to choose the spelling that best captures the sound you're imagining.