I had heard that too. They crashed a satellite into it and it reverberated for a while

After Neil Armstrong and Buzz Aldrin had transferred back into the command module with Michael Collins for the return trip to Earth, the decision had been made at NASA to de-orbit the LM ascent stage back down to an impact on the lunar surface in order to test the remote seismometers that had been put in place at the landing site. An impact in a known location, of known mass, velocity, and therefore energy, would give them a good signal for test and calibration purposes.

Upon impact of the LM ascent stage, the signals sent back by the seismometers surprised all of the geology experts, because the signal it generated would have been more consistent with an impact on a hollow object, causing it to reverberate, like a bell, than a solid object. In fact, this "ringing" signal continued to reverberate for several hours. This signal has caused some to theorize that the moon is hollow.