I could not believe it either. I was visiting San Francisco several years ago, riding a bicycle across the bridge when I see a red 407 heading for the bridge. All of a sudden the heli starts to dive and I knew he was heading under the bridge! Sure enough, that is what happened. I was amazed! Now I live here in San Francisco and I see this almost weekly.
To confirm it's SF Helitours I found another clip. Tour starts at 1:05, under bridge at 3:20.