Definition of "pinging"

[…] walking across the open in stern composure, with the pingings of the hostile bullets, and the imploring cries of his comrades to save himself, sounding in his ears.

1870, Sir John William Kaye, A History of the Sepoy War in India, 1857-1858, volume 2, page 320

When she entered, her eyes and ears were assaulted by neon lights in bright colors, the hypnotic, musical pingings of electronic slot and poker machines, and the clinking of coins dropping into payout trays.

2011, Ramona DeFelice Long, Fish Tales: The Guppy Anthology, page 202