Need two players, one Amphibia S.
One player fires Amphibia S without a bullet in the chamber.
Other player moves away to confirm the how far the clicking sound can be heard.

When this occured my Amphibia S was bugged, it had no bullet in the chamber but no bullet could be chambered either (as it thought there still was a bullet inside). However after chambering a few bullets and firing it unloaded the sound still persisted.