If you've heard it then I'm sure you know what I mean. In your experience where was the animal hit to create this sound? Is it an indication of a solid well placed hit or not?
I've been hunting for 55 years and killed over 50 deer. In my experience, that sound means nothing except a hit. It is the sound of a bullet hitting flesh. The pitch of the sound is more related to the caliber than it is to the nature of the hit.
A pellet gun makes a whack sound when it hits a startling. A varmint rifle makes a whump sound when it hits a gopher or a coyote. A deer rifle makes thwaruck when it hits a deer or a coyote, and so on. I've never seen or noticed any correlation between the hit location on the game and the sound.
Just my two cents for whatever it's worth.