In the army I had the opportunity to fire the Carl Gustaf 84mm recoilless rifle, which is normally a two person job (one to hold, aim and fire, the other to load and check the backblast area).

We're instructed to exhale before firing because the concussion of the round leaving the front while its propellent leaves the back of the tube creates a brief vacuum. If you don't exhale, the air is forced out of your lungs so violently you feel like you got punched in the chest.

The noise is undoubtedly part of it, but the atmospheric effect is not insignificant, I think.