That "ping" noise gave rise to one of the silliest stories told during and after WWII. At some time or other, some armchair general heard about that "ping" and invented a story that goes something like this:
When the last round is fired from an M1 rifle, the ejected clip makes a pinging sound when it hits the ground. The enemy listened for that sound. Knowing the poor American was struggling to reload his rifle, the German (or Japanese) would charge over the 100 (or 500 or 1000) yards of ground separating the lines and bayonet the GI in his foxhole while his rifle was empty and he was helpless.
Of course this requires 1) that the battlefield be so quiet that the ping can be heard 100 (or 500 or 1000) yards away, 2) that the enemy soldier is in good enough physical condition to run 100 (or 500 or 1000) yards, 3) that it takes a long time to reload an M1, and that 4) the two soldiers are alone on the battlefield. If anyone believes any of the above, he might be gullible enough to believe the silly "ping" story.
(FWIW, a man having experience with an M1 and with clips handy can reload almost before the clip hits the ground, so that enemy soldier might have a problem.)