As soon as you enter the tunnel, you will hear a guard ask if this is the only shipment. Do a full sprint towards the guard and the truck that is leaving. Duck into the doorway on the right side and quietly open the door. If you do not do this at a full sprint, a second guard near the truck will see you as the truck pulls off and set off the alarm.
The guards at the bridge and the tower can be taken out. You have to kill the tower guard first, then spy on the guards on the bridge until both of them are looking away from you. Then, run up and backstab each of them.