From my experience there is one factor that makes a shell quiet, and it's not Teflon tape.
I am referring to a normal expanded shell, or a non-expanded shell; sliding half shells (i.e. Slippery Sams) are another story.
The sound of de-shelling and re-shelling a coin is made from the coin scraping the inside rim. Logically: a shallow shell makes less noise than a deep shell.
Lining the inside bottom with Teflon would not cut down on shell noise. Teflon lining also give an extra layer of material a magnet must work through for a shimmed shell, this would give it a weaker magnetic pull.
The way I see it:
Deep shells = more noise
Shallow shells = less noise
Of course deep shells look nicer, but sound louder.