I'd not heard this in reference to the European front (I'm sure it was probably the case) but on the Pacific front, infantry guys used to carry empty garand clips with them, fire about 3 shots, throw the clip on a hard surface to make the "ping", the japanese soldiers would come out of hiding and charge thinking the americans were reloading, then the americans would let them have it.
Just a tidbit if you never heard it before =)