I was just about to write about this when I read your post.
This trick was also used against the Germans as well. It seems upon hearing this ping the germans knew the soldier was out of ammo and would be reloading so they would use that moment to rush his position.
the GIs would use that trick to fool the enemy into charging then blast away at them while they were in the open.
Or as you said raised their head/exposed themselves for a better shot.
LOL classic example of "when life gives you a lemon. Make lemonaid"
Originally posted by Kommandant
The Ping of an Ejecting empty clip. Several GIs in Both theathers started carrying around extra empty clips and after a couple rounds fired, they would Ping the clips together. My Grandfather was one that did just this. He killed several Japanese when they raised their head to get a better shot this way.