yikes, my (non-professional) experience with these is probably 35 years back, but the ones I recall had a little transformer/solenoid type thing, with plungers that would strike the flat bar chimes when energized: 
If it’s simply a matter of the plungers being gummed up, and the solenoid “hums” when the doorbell button is pushed, the trick was to lube the plungers with good, old fashioned, lighter fluid:
As I recall, this was actually advised on a little sticker inside the door chime housing.
Mike