Personally, I hated my Tetra Whisper PF10. It was actually louder than my Aquaclear that I bought that was rated for 30 gallons. The reason why yours is quiet would be because the motor is in the water, reducing the noise, or at least reducing the noise out of the water... Not sure about inside.
To me those cartridges are a waste though. I like how the Aquaclear comes with 3 stage filtration and adjustable flow.
I just got the Marinara 10, suitable for like 5 gallons, it has the motor inside as well, but is a hang on back. It is nice, though they waste filtration space in how you set up the cartridges. Also, the adjustable flow on the lowest setting barely even moves the water, while the high setting does a good job.
I still like Tetra products, but filters that have that adjustable flow is nice.