From "Iron Coffins" by Herbert A. Werner (p.293):
... However, the thunder of the barrage was not nearly as frightening as the noises that preceded and followed the attack. At first it sounded as if something like a chain or an iron net were being dragged along the hull; then came a wild clattering, as if a great load of pebbles were bing dumped upon the boat. These were brand new sounds to all of us - and a terrifying turous experience... But I concluded - correctly, as I later learned - that the British were using a new search device akin to the Asdic.
If I've got the dates right, that would've been March 19th, 1944. I imagine that at some point prior to that, the Allies started playing around with sound frequencies higher than previously used, causing the shift from the high pitched ping that could be heard earlier to that "load of pebbles" sound.