Very very simple this one.
Start up Officer Hunt in Loue Valley and do the first bit (chance to get It's a Trap) then proceed onto the next part of the map where you will see a sort of checkpoint with a guard tower and a half track. when facing that point look to the right and you will see a track leading up a small hill, go up here and equip your Sniper (left or right on the d-pad)
Snipe the gunner in the half track and reload checkpoint. do this twice more and you will get the achievement also if you wait until the noise meter appears in the top right corner this kill will also count towards the Loud Sound achievement.
hope this helps :)