Its really awakward when anyone face this situation,but when she farts you need to pretend that you haven’t heard anything , you need to make her comfortable.
If she laughs, laugh with her and maybe offer to start over with the proposal–which could be both funny and romantic. If she doesn’t laugh, maybe proceed as if it didn’t happen.
Having said that, if a lady farts when you propose to her and you aren’t able to laugh about it together, your relationship may not be ready for the commitment of marriage.
By the time you propose marriage, you probably want to be comfortable enough around each other that such a minor embarrassment won’t create awkwardness. In marriage, you will have frequent reminders that neither of you are perfect. It will be valuable for you to be able to handle accidents and mistakes without becoming mortified or indignant.
Humour is a much better sign of a good relationship than cookie-cutter romantics. If you both laugh good-humoredly at this situation, you’re probably doing pretty well.