My Ford Fiesta passed it's MOT last month but when I brake to almost a complete stop my car it makes like a whooshing sound?

Sounds like the brakes. It sounds worse in a morning when I start using the brakes. Is this normal?

Vacuum leak.

The power brakes run off of engine VACUUM, so probably OK.

If there had been a problem at the time it would have failed the mot… If the brakes work as well then it does not sound like the vacuum pipe. If it was leaking you would need a lot more force… Regardless don't ask here. Get it to a garage

Something is wrong with the booster. Take your car to a garage.