Related to the Lingering CoC, the CoC of Heart..
Whatever happened to Sommers' Heart succeeding Zoro's final impact blow..
The Mechanical Heart echoed all the way to Franky who as a Cyborg is more attuned to that wavelength..
The sound effect in Japanese is '' Gakon '', which is used to represent a sharp, heavy mechanical clunk or metal impact..
You can see Franky feeling this sensation from only one side like a tingling but with metal..
Similar to Hody reacting to Luffy's CoC wave in fishman Island..