I've found Whispering Snake Skulls in Old logs/stomps, but never in butchered Timber Rattlers.
As WIKI informs, Timber Rattler has WSS:
Random chance to get various rare items from butchering. Chance of dropping is much higher with Viscera & Bits.
This includes: Etterfang, Forked Tongue, Rattle Rattler, and Whispering Snake Skull.
As for me, I killed many these snakes, looted about 10 Etterfangs but never WSS. Anybody lucky?