The key gap may also be a variable. The wider the gap, the more distance to travel and the faster the terminal velocity. The faster the key's terminal speed on releasing, the louder the noise.
Also, a wider gap would give the impression of a heavy spring as you're using more of the spring's tension.
To alleviate all of these variables for the "perfect" feel, the felt material, the gaps, the spring tension could generally be tweaked by a tech in very little time.
But as I said in the other thread, your best bet for peace of mind would be to get a full "set up" done by your local tech, explaining this key noise is something you've noticed, and for a small outlay you'd have a good as new horn with no worries for a good while.
I recall once having an annoying vibration I couldn't track down on an alto. Took it to a tech, Paul Carrington, and played a few notes.
He placed one finger on a rod and the noise stopped. It was a missing rod cup felt. Sometimes it takes years of experience to spot the obvious.
A good tech is worth their weight in gold. Let's hope the one near you is a good one!