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Currents at the bottom could be bouncing the wrecked submersible against the hull of the Titanic, making a banging noise.

In the 1959 movie "On the Beach" all human life in the northern hemisphere has been killed due to a nuclear war a U.S. Submarine was sent to see if the radiation is dispersing and to find the source of a radioed telegram signal.

(From IMBD) "The submarine next stops at a refinery near San Diego, which has been pinpointed as the source of the mysterious Morse signals. A crew member discovers the power source is still running on automatic control. Nearby, a telegraph key has become entangled in a window shade's pull cord and a half-full Coca-Cola bottle, and is being randomly pulled by an ocean breeze, causing the radio signals. "

Something like that.