Imac Pro, "change volume" sound is strange

Hello all,

I'm new in here.

here's what happens: I have an iMac Pro with latest High Sierra OSX. The "play feedback when volume is changed" in "audio" is checked and when I change the volume higher and/or lower, I hear the normal sound plus another sort of click sound layered together. This happens when I change it slowly, if I change the volume pressing a little bit faster I only have the normal sound appearing. This is strange and I don't know how to be more precise about the description.

Does it happens also to someone? Maybe one with the iMac Pro?

I hope you can help me.