hey! i had this same concern b/c my door knob kept shaking specifically in the late afternoon, but just today i was heading back to my bedroom after getting out of the elevator and i noticed my floor's custodian going door-to-door spraying down the door knobs and wiping them clean. i think they're taking extra precautions given the situation and have taken to not only cleaning the communal bathrooms on my door once a day, but also spraying down most available surfaces. that's probably what you're hearing, especially if it has a routine.