おならの音の「ブッ」とか「ブッー」って英語でなんて言うの？

This is a sound that you make by putting your tongue between your lips and blowing so that your tongue vibrates. People will do this when they want to trick someone into thinking that they or someone else is farting. I hope this helps :)

There are many words for passing gas. However, I wouldn't say many people use these words when talking about farting. They would say, blow off, trump, popped, etc. These words sound a little like the sound but not entirely but you would hear them in conversation.

If someone farts quietly it is known as 'fluffing' which is onomatopoeic.

Onomatopoeia - is the sound something makes but when talking about farting there is no exact word used the best way to describe what it sounds like it by putting your tongue between your lips and blowing or a pfft sound although there are many words to describe farting itself (trumping, blowing off, passing wind etc)

Passing gas sounds like: Pfft. However, when talking about the subject it would be unlikely that a person would actually say "Pfft". That is more likely to be said by children being silly. A teenager or an adult would use words such as: Farted Blew off Guffed Sharted - this is generally used when someone things they may have followed through (defecated) when they farted. The word is a combination of the words shit and farted.

The onomatopoeia you're looking for is usually "Pfft", but often enough it can be something else. Such as: "whoof", "flurp", "poot", etc. Most of these make more sense when you try to pronounce then in a funny way.