Anti-Mage's Mage Slayer set comes with many animations, including a custom rare idle animation. This animation is him performing a backflip. It is completely silent currently.
As a comparison, Troll's default rare idle animation is a front flip. It has a flip sound and a landing sound. The same sounds could be used for the Mage Slayer set's rare idle animation.
The spin sound is under sounds\misc\idles\idle_troll_spin.vsnd
The landing sounds are the default footstep sounds under sounds\physics\footsteps\hero\shared\general01-07.vsnd