If your vet isn't concerned, I wouldn't worry about it. Buster does this every time I lift his hind leg. Just relaxing to him, I guess.
Now I know another horse on our farm that does have much more serious gas problems due to his feed. He has to have a special, pelleted feed. If he eats hay, his gas is terribly explosive. You can hear it all the way across the farm and it goes on constantly. It must be very painful for him. This is no laughing matter, of course, so he stays on his special feed most of the year.
Life is hard enough on its own, please be kind.