At one side of the map there should be a tank with a few rockets on.
This is on the same side of the map as the rundown chopper.
To the right of the tank there should be a blown up car, jump on top of it and get as far back as possible for your big run up.
Now there should be a few cinder blocks as you face the tank, sprint towards them and so on until you get to the tank and then simply jump on the tank and up to the rockets.
This can be done in ranked or private matches and is an excellent hiding place.