Very good, ghostpotion! I don't claim to know anything about sound, although I was familiar with harmonic frequencies in my Navy career. My guess - and it's only a guess - is that the stiffness of the rail consolette set up a harmonic vibration that vibrated the slightly loose washer...but the relative "softness" (the "give") of a snare stand and - same as holding the tom in your hand - that vibration/that frequency was not "excited."

Yes, always a Gremlin or two in vintage drums! Ghosts of former owners/players?! Or the ghosts of those who built and assembled the drums?!

Glad you found it!

Regards, mb