A periscope vibrating is basic physics at work, think of a flagpole in a high wind. An extended scope is a long metal tube, metal, but still flexible. As the sub moves forward, water pressure pushes the tube aft, but you can only bend the tube so far before it snaps back to its original position at which point the process restarts. Of course in RL, the effect is more dynamic, but the result is that the faster the sub goes, the higher the vibration effect.
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