I've have had this issue with aftermarket GTB and particularly WMS hold coils. I have found that a lot of the 'chattering' / buzzing noise is greatly reduced when you file the iron end of the coil perfectly flat and fit a new or good armature so that both mating surfaces are perfectly flat. I also fit a new armature spring too.
I've read the diode/capacitor idea too so might try that if I just cannot keep a coil quiet.