Why were there several unidentified sounds detected by NOAA in 1997, but few before or after?

Question

NOAA detected “Whistle” on July 7, 1997, “Bloop” in the summer of 1997, “Slow Down” on May 19, 1997, and “Train” on March 5, 1997.

4 of the 6 notable unidentified sounds occurred within a 6 month period in one year, with no signal as significant, or at least not as many signals as significant, reported before or since.

Can someone explain this?

In: Planetary Science