Bayonets almost always rattle due to the loose tolerances required for rapid fixing (mounting) and un-fixing (dismounting) of the blade on the rifle/carbine. That rattle of fixed bayonets can be put to good use for intimidating unruly crowds. I recall when we conducted anti-riot training for Aide-of-the-Civil-power operations in the Regular Force Infantry (PPCLI) back in the late 1980s and 1990s we would incorporate a slap of the FNC1A1 (later C7) Handguards every time the Left foot hit the pavement during the formation advance with bayonets fixed. The resultaing metallic rattle of the bayonets was quite loud and tended to cut through the din of the riot to good effect. Such noises tend to focus the attention of those liable to be on the receiving end of "cold steel"!!
There is still very good reason for bayonets on rifles and carbines wielded by the Combat Arms. And not just for riot or POW escort duties. I personally saw soldiers fix bayonets while manoeuvring through heavy-growth grape fields under fire, or while clearing through objectives with suspected enemy combattants, as recently as my last tour of Afghanistan in 2008. Not sure if it is still SOP as part of Offensive Battle Drills to fix bayonets and change magazines just prior to the Assault, but it always made good sense to me. Far better to have a stab-capable "lance" than a "club" in your hands, should you suddenly encounter an enemy combattant at close range while reloading, eh? Bayonets continue to serve a multitude of useful purposes outside of their primary role in close combat. Besides all of that, they scare the living sh#t out of the enemy, all of whom are far more terrified of being put to the blade than the bullet. Bayonets truly are a terrific close-combat-multiplier. Great stuff!