Our dog Boudreaux farts often, sometimes silently, but sometimes with a loud rrrrrriiiiiiiippppp sound as his anus vibrates. His burps are always audible, however.
Our dog Scarlett always farts silently but she snores, grunts and emits an eerie hum when she sleeps. I have never heard her burp.
Our dog Abigail used to produce truly evil and incredibly loud farts, particularly when she would lie at my father’s feet after dinner, with her butt facing him. She would always leap to her feet and whip around to see who did that. She often ended up staring accusingly at my father, whose face was slowly turning green from the intensity of the odor.
Our cat Mercy farts silently and only when his butt is directly in front of your nose.
Our cat Outdoor Kitty farts with a sound rather like a single purr. It always startles him so he leaps up and whirls around looking for the source of the noise.
It seems to depend on the individual animal and the back pressure involved.