Having a Crossbow in the Hands slot will play a swinging sound, akin to the sound you hear when attaking with a melee weapon, for each step taken.

The frequency of the sound inceases when holding the Sprint modifier while crouchwalking. Further implying the sound is linked to the steps.

It plays with all 3 loading states of the crossbow. Charged without bolt, charged with bolt, and neither charged nor loaded. I have not been able to thes the impact of the durability level.

Crawling and regular walking or sprinting does not play the sound.

All 4 directions (W,A,S,D) causes the sound to play.