Are bonded stainless pans safe to use if they make knocking sounds?
Are they safe? Sure. I think you might be thinking of thermal stress in something like a glass casserole dish, where a temperature gradient can cause the thing to violently fracture, exploding in all directions. (Trip report: It was surprising, and terrifying in retrospect!) That can't happen with a metal pan, which will deform rather than fracturing.
Moreover, the stress on your partially separated pan is less than it would be on a non-busted pan. The sounds you're hearing are the sounds of stress being released little by little, rather than being stored up.
A bonded pan bottom comes off bit by bit over a large number of heating cycles, and in my experience never comes off completely (unless pulled off). But once the separation starts, thermal conduction goes way down in the separated areas, leaving uneven hot and cold spots. So these pans aren't going to kill you... but they're likely to disappoint you.