My Mysterioso build sounds REALLY good. By far the coolest 4049 build I've heard. I have a fairly noticeable buzz when the bottom toggle is in REACT mode. With the buffer in circuit it's dead quiet. It's almost like I lose the ground but it's not that loud. I'd almost expect it to be the other way around.
I guess I'd start with swapping out the 4049 but the fact that just putting a buffer in front of it has me a bit stumped. This hasn't been boxed yet so not sure if the extra shielding will bring down the noise.
Not sure what would cause that other than perhaps general noise from not being boxed, or possibly if you are using a breadboard to make connections while testing. Obviously the reactive mode input is completely passive so that should not be a source of noise. Maybe do some continuity checks on the actual DPDT switch itself and make sure the connections are solid in both modes.