Quoted from Turboderf:the only thing I don't like about FT is the topper I put a small switch to turn off the fishs tale
Alternatively, consider placing a 5/16" ring on the bell part of the armature.
Or put a post rubber over the plunger and roll it down to the base and reassemble.
Some people report being happy with the quietening results.
Depends on your personal flap tolerance.