You really can't tinker with the gun without a chrony. Without fps feedback, you'll be chasing your tail.
The transfer port screw, spring tension, and striker distance are all adjusted independently. When tuning, it's usually best to adjust one at a time. In my experience - the port screw is the master controller for potential power. The spring tension is then adjusted to get the most out of that power. And the striker is adjusted for fine tuning the shot string. Others do things differently...
The gun does make more sound when tuned to shoot with higher energy. But, even more, the sound varies from pellet to pellet. A CP at 800fps is louder than a JSB 18 at 850fps, despite having much less energy. The "hard" pellets like CP and CPHPs make a "ting" sound when they fire. JSBs, Kodiaks, and Baracuda have more "air" sound to them, and less "ting".
IME, JSB's fired at 80-90% max power (800-880fps), along with a depinger, makes for a damn quiet bunny killer. That said, don't be obsessed with how quiet you can make the gun. It's already quieter than my crosman 1377 - with 1/4 the power.
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