All my .177, .22, and .25 springers are loud compared to a Marauder.
I have the Marauder in .22 and .25 calibers.
The 22 Mrod is really, really quiet...and as someone just said, you may think that your mrod malfunctioned...but it did not!!!
I've done tests where my wife shoots the 22 mrod in the backyard, while I listen in the front yard...I heard nuthin!!!
The 25 is not anywhere near as quiet as the 22.
When I listen for the 25 mrod in the front yard, I can hear like an air stapler.
I dont know about other PCP rifles, cause I dont have any others.
My vote for you goes to the 22 Marauder...you'll be amazed.
The thing about measuring pellet gun sound, to measure the sound accurately, you need a very expensive SPL(sound pressure level)meter.
This type of SPL meter has an "Impulse"(35 milliseconds)response time setting.
The affordable SPL meters have a "Fast"(125 milliseconds) and "Slow"(1 sec)response time setting.
We need somebody who has a SPL meter with an Impulse setting and some PCPs to test.
I've got a nice SPL meter, but it doesnt have the impulse setting, just the typical fast/slow.